Vehicle braking system distribution of front and rear braking forces is controlled according to two different distribution patterns depending upon vehicle load

ABSTRACT

A braking system for braking a motor vehicle by the front and rear brakes of the vehicle&#39;s front and rear wheels, respectively, including a distribution control device for controlling a distribution of front and rear wheel braking force produced by the respective front and rear brakes, according to a selected one of a first and a second distribution pattern, each of which represents the front and rear wheel braking forces in relation to each other wherein the rear wheel braking force defined by the second distribution pattern is larger than that defined by the first distribution pattern at least when the front and rear wheel braking forces are smaller than respective predetermined values. The selection of the two distribution patterns may be dependent on vehicle load.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a braking system, and moreparticularly to techniques for improving braking performance of a motorvehicle's braking system by optimizing the distribution of brakingforces between the front and rear wheels of the vehicle.

2. Discussion of the Related Art

FIG. 1 illustrates the braking forces of a motor vehicle's front andrear wheels as an upwardly convex curve where the front and rear wheelsbegin to lock simultaneously on a road surface. In FIG. 1 the brakingforce of the rear wheel is plotted on the vertical axis while thebraking force of the front wheel is plotted on the horizontal axis. Thiscurve, referred to as an "ideal distribution curve," represents an idealdistribution of the braking forces applied to the front and rear wheels.To improve the braking capacity or performance of the braking system,the specifications of the braking system should be optimized so that anactual distribution curve, representing the actual distribution of thefront and rear wheel braking forces, is as close as possible to theideal distribution curve. The specifications of the braking systeminclude, for example, diameters of the front and rear wheel brakecylinders, effective radii of the front and rear disc brake rotors, andinside diameters of the front and rear wheel brake drums.

While the ideal distribution is represented by a curve as describedabove, the actual distribution of the front and rear wheel brakingforces, as established by the basic arrangement of a braking system, isrepresented by a straight line, as indicated in FIG. 1. The basicarrangement is not provided with a load-sensing proportioning valve (LSPvalve). It should be noted that the rear wheel braking force is notconstant, but increases as the amount of load acting on the vehicleincreases with respect to the minimum load (i.e., a load acting on thevehicle during a minimum-load run of the vehicle). A "minimum-load run"means a run of the vehicle with only the driver (without any passengers,in the case of a passenger car, or without any cargo, luggage or load,in the case of an industrial vehicle). FIG. 1 further illustrates thatthe ideal distribution curves during a minimum-load run of the vehiclediffers from that during a full-load run of the vehicle. The "full-loadrun" means a run of the vehicle with the nominal number of passengers(including the driver), in the case of the passenger car, or the nominalmaximum load, in the case of the industrial vehicle.

Further, upon braking of the vehicle the braking system is generallydesigned to avoid locking of the rear wheels prior to locking of thefront wheels to prevent the vehicle from losing control in the runningdirection. The braking system is also designed to prevent locking of therear wheels prior to front wheels during the minimum-load run where theload acting on the rear wheels is the smallest, and corresponding to thehighest locking tendency of the rear wheels. Described in greaterdetail, the braking system is usually adapted to minimize a deviation ofthe actual distribution of the front and rear wheel braking forces(i.e., a deviation of the basic distribution line as determined by thespecifications of the most basic braking arrangement indicated above)from the ideal distribution curve in a direction that causes an increasein the rear wheel braking force.

In practice, however, it is difficult to design a braking system with anactual distribution line of the front and rear wheel braking forces thatis sufficiently close to the ideal distribution line. As shown in FIG.1, the actual distribution line has a larger amount of deviation fromthe ideal distribution curve during the full-load run of the vehiclethan during the minimum-load run. The deviation is attributable to thedifference between the ideal and basic rear wheel braking force values.In other words, designing braking systems to accurately follow the idealdistribution curve is limited since the actual basic distribution isgenerally represented by a straight line and the ideal distributioncurve varies as a function of vehicle load.

While the basic arrangement of the braking system has the drawback asdescribed above, an improved arrangement also exists where aproportioning valve is disposed between the hydraulic pressure sourceand a rear wheel brake cylinder. This arrangement results in an actualdistribution line which is closer to the ideal distribution curve. Asindicated in FIG. 1, the actual distribution lines of the proportioningvalve (load-sensing proportioning or LSP valve) are bent straight lineswhich are closer to the ideal distribution curve than the basicdistribution lines. As disclosed in laid-open Publication No. 2-130870(published in 1990) of unexamined Japanese Utility Model Application,the proportioning valve is a pressure reducing valve that is adapted toreduce the hydraulic pressure generated by the hydraulic pressure sourceat a predetermined ratio and apply the reduced hydraulic pressure as thebraking pressure to the rear wheel brake cylinder after the generatedhydraulic pressure has exceeded a predetermined threshold level. Untilthe generated hydraulic pressure reaches the threshold level (indicatedby dots in FIG. 1 at the points of bending of the actual distributionlines of the proportioning valve), the proportioning valve does notfunction as the pressure reducing valve, and the hydraulic pressuregenerated by the pressure source is applied to the rear wheel brakecylinder.

In industrial vehicles, such as trucks where the load acting on the rearwheels varies considerably as the amount of cargo varies, the brakingcapacity or performance is insufficient when the load on the rear wheelsis relatively large and the threshold level indicated above is fixed(i.e., if the level of the generated hydraulic pressure at which theproportioning valves begins to function as the pressure reducing valveis fixed). In view of this drawback, the braking system for suchindustrial vehicles is equipped with a load-sensing proportioning valvealso known in the art. In the load-sensing proportioning valve(generally referred to as "LSP valve", or "LSPV"), the threshold levelwhich corresponds to the point of bending of the distribution line ofthe valve varies as a function of in the amount of load on the vehicle.There are two types of load-sensing proportioning valve: linkage andball. The linkage LSPV utilizes the fact that the amount of relativedisplacement between portions of a sprung member and an unsprung memberthe rear wheel assembly increases with the load that acts on the rearwheels. Thus, the linkage LSPV is adapted to detect the vehicle load inthe form of the relative displacement amount of the sprung and unsprungmembers. The ball LSPV utilizes the fact that the rear portion of thevehicle body is raised in relation to the front portion as the load onthe rear wheels decreases. The ball LSPV uses a ball adapted to roll onan inclined surface wherein the inclination angle changes with theinclination angle of the vehicle body, so that the ball is seated on avalve seat as a result of rolling. In the ball LSPV, the difficulty ofrolling of the ball on the inclined surface indicates vehicle load.

However, the degree of approximation of the distribution line of theload-sensing proportioning valve to the ideal distribution curve islimited. That is, it has been difficult to sufficiently solve theundesirable tendency that the actual distribution line of theload-sensing proportioning valve deviates from the ideal distributioncurve, in the direction that causes the rear wheel braking force to besmaller than the ideal value. This is particularly problematic when thevehicle is in the full-load run. As illustrated in FIG. 1, the hatchedarea is an area of deviation of the actual rear wheel braking force fromthe ideal value. Therefore, during the full-load run of the vehicle, theactual braking forces applied to the rear wheels are considerably lowerthan the ideal value, or cannot be increased to the optimum value. Thus,the use of load-sensing proportioning valves still suffers frominsufficient rear wheel braking forces, although rear wheel locking isprevented.

The above-identified problem of increasing the rear wheel braking forceto the ideal or optimum value during the full-load vehicle run alsoexists in known anti-lock braking systems adapted to control wheelbraking pressures. The anti-lock control of braking forces will bedescribed below in detail.

Braking systems are classified into two types: independent front-rearbraking force control and diagonal or X-crossing. In the independentfront-rear braking force control type, the first pressure applicationsub-system, including the front right and left wheel brakes, isindependent of the second pressure application sub-system, including ofthe rear right and left wheel brakes. In the X-crossing type, the firstpressure application sub-system, including the front left wheel brakeand the rear right wheel brake, is independent of the second pressureapplication sub-system, including the front right wheel brake and therear left wheel brake.

In an anti-lock braking system of the independent front-rear brakingforce control type, the front and rear wheel braking pressures areusually regulated independently of each other during anti-lock brakingpressures control. In this case, the actual front-rear distribution ofthe braking forces is not bound by the basic distribution linedetermined by the specifications of the braking system, but can bechanged with a high degree of freedom from the basic distribution line.Accordingly, the actual distribution line can be made sufficiently closeto the ideal distribution curve. Therefore, the braking system of theindependent front-rear braking force control type does not suffer fromthe above-identified problem that the rear braking forces cannot beincreased sufficiently during the full-load run of the vehicle.

In an anti-lock braking system of the X-crossing type, severalarrangements are available for anti-lock control of the braking forces.One example of such arrangements is illustrated in FIG. 2, wherein anormally-open master cylinder cut-off valve 306 is provided in a frontbrake cylinder passage 304 connecting a master cylinder 300 (hydraulicpressure source) and a front wheel brake cylinder 302, while a rearbrake cylinder passage 308 is connected at one end thereto to a portionof the front brake cylinder passage 304 between the cut-off valve 306and the front wheel brake cylinder 302. The rear brake cylinder passageis connected at the other end to a rear wheel brake cylinder 307. Anormally-closed shut-off valve in the form of a pressure reducing valve312 is provided in a reservoir passage 310, which is connected at oneend thereto to the rear brake cylinder passage 308 and at the other endto a reservoir 316. The reservoir 316 which receives the brake fluiddischarged from the wheel brake cylinders 302, 307 through the shut-offvalve 312. A pump 318 is connected to the reservoir 316 to return thebrake fluid to the master cylinder 300. According to this brakingarrangement, the braking pressures in the front and rear wheel brakecylinders 302, 307 are increased by the pressure generated by the mastercylinder 300.

The assignee of the present application proposed another brakingarrangement of the X-crossing type, as shown in FIG. 3. Unlike thebraking arrangement of FIG. 2, the present braking arrangement of FIG. 3is adapted to increase the braking pressures in the front and rear wheelbrake cylinders by operation of the pump 318. That is, the mastercylinder cut-off valve 306 is held closed during an anti-lock control ofthe braking pressures and the pump 318 is connected to a portion of thefront brake cylinder passage 304 which is downstream of the cut-offvalve 306. Correspondingly, the pressurized fluid from the pump 318 isnot returned to the master cylinder 300 but is returned to the wheelbrake cylinders 302, 307, whereby the braking pressures in the wheelbrake cylinders are increased by operation of the pump 318 during theanti-lock control of the braking system.

In either of the two arrangements of the anti-lock braking system of theX-crossing type, the braking pressures in the front and rear wheel brakecylinders 302, 307 cannot be regulated independently of each other, butare regulated such that the braking pressure in the front wheel brakecylinder 302 is equal to that in the rear wheel brake cylinder 307.Therefore, unlike the braking system of the independent front-rearbraking force control type, the braking system of the X-crossing type isnot capable of establishing the actual distribution line which isshifted from the basic distribution line in the direction that causes anincrease in the braking pressure in the rear wheel brake cylinder duringthe anti-lock control. Thus, like the ordinary braking system incapableof effecting the anti-lock control of the braking forces, the brakingsystem of the X-crossing type suffers from the problem of insufficientrear wheel braking force during the full-load run of the vehicle.

Further arrangements of the anti-lock braking system of the X-crossingtype are illustrated in FIGS. 4 and 5. In the arrangement of FIG. 4, two3-position valves 320 each having a pressure-increase position, apressure-hold position and a pressure-decrease position are provided forthe front and rear wheel brake cylinders 302, 307, respectively. In thearrangement of FIG. 5, a series connection of two shut-off valves 322,324 is provided for each of the front and rear wheel brake cylinders302, 307, in place of the 3-position valve 320 used in the arrangementof FIG. 4. Although these arrangements of FIGS. 4 and 5 permit theactual distribution of the front and rear wheel braking forces to becontrolled without restriction by the basic distribution line, thesearrangements suffer from the separate problem inevitably complicatedconstruction, which corresponds to increased manufacturing cost.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to provide abraking system for a motor vehicle, which is improved in brakingperformance over the prior art braking systems. In particular, in itsability to increase the rear wheel braking force during the full-loadrun of the vehicle by selectively using two distribution patterns whichrepresent front and rear wheel braking forces such that the rear wheelbraking force defined by one of the patterns is larger than that definedby the other pattern. Correspondingly, the actual distribution is asclose as possible to the ideal distribution curve.

It is a first optional object of the present invention to provide abraking system wherein the two different distribution patterns areestablished by a device which is simple in construction.

It is a second optional object of the present invention to provide abraking system which permits optimum distribution of the front and rearwheel braking forces by selective use of the two different distributionpatterns that correspond to ideal distribution value of a relativelysmall and large values of the vehicle load respectively.

It is a third object of the present invention to provide an anti-lockbraking system of a diagonal or X-crossing type having two pressureapplication sub-systems each basically designed to effect simultaneouscontrol of the front and rear wheel braking pressures. The correspondingbraking system permits improved anti-lock control of the brakingpressures during a full-load run of the vehicle, in particular, the rearwheel braking forces are larger than the front wheel braking forces,while avoiding complicated construction of pressure control valvedevice.

It is a fourth optional object of the present invention to provide ananti-lock braking system of a diagonal or X-crossing type having twopressure application sub-systems each having a fluid recirculating pumpoperated to increase the front and rear wheel braking pressures in ananti-lock pressure control mode and an intermediate valve for permittingonly the front wheel braking pressure to be increased. This assuresoptimum distribution of the front and rear wheel braking forces.

It is a fifth optional object of the present invention to provide ananti-lock braking system of the X-crossing type where only the frontwheel braking pressure can be increased, and that uses a simplyconstructed check valve device.

It is a sixth optional object of the present invention to provide ananti-lock braking system of the X-crossing type where only the frontwheel braking pressure can be increased, and that has a duty-cyclecontrol mode wherein an intermediate valve is alternately closed andopened to control the front and rear wheel braking pressures, with anincreased freedom of control.

It is a seventh optional object of the present invention to provide ananti-lock braking system wherein the duty cycle of the intermediatevalve can be changed to thereby further improve the freedom of controlof the braking pressures.

It is an eighth optional object of the present invention to provide ananti-lock braking system wherein the duty cycle of the intermediatevalve can be optimally controlled on the basis of the pressure reducingtendencies during the anti-lock pressure control operation.

The above-identified principal object may be achieved according to theprinciple of the present invention, which provides a braking system asschematically illustrated in FIG. 6, for braking a motor vehicle byoperation of a front brake 202 and a rear brake 204 for respective frontand rear wheels of the vehicle. The braking system includes adistribution control device 210 adapted to control a distribution ofbraking forces applied to the front and rear wheels and that is producedby the front and rear brakes 202, 204, respectively. The distributioncontrol device 210 controls the distribution according to a selected oneof either a first distribution pattern or a second distribution pattern.Both the first and second distribution patterns each represent the frontand rear wheel braking forces with respect to each other. In particular,the rear wheel braking force defined by the second distribution patternis larger than that defined by the first distribution pattern at leastwhen the front and rear wheel braking forces are smaller than respectivepredetermined values. Namely, the first and second distribution patternsare formulated so that the rear wheel braking force according to thesecond distribution pattern is smaller than that according to the firstdistribution pattern, at least when the vehicle is braked with therelatively small braking forces applied to the front and rear wheels.

The braking system of the present invention applies to friction frontand rear brakes, as well as other types of brakes such aselectromagnetic brakes, rheostatic brakes, electro-regenerative brakes,and pneumatic brakes.

The distribution control device 210 used in the present braking systemmay take various forms. In the braking system equipped withhydraulically operated friction brakes, for example, the front and rearbraking forces may be controlled by regulating the hydraulic brakingpressures (pressures of the brake fluid) applied to the friction brakes,or by controlling pressure-receiving areas of pistons that receive thehydraulic braking pressures to force friction members onto rotorsrotating with the vehicle wheels. In the latter case, the number of thepistons required for each of the rotors to effectively operate may beindividually selected.

In one form of the distribution control device adapted to regulate thehydraulic braking pressures, the front wheel braking pressure and therear wheel braking pressure are controlled to have the same levelaccording to the first distribution pattern, while the front wheelbraking pressure is reduced with respect to the rear wheel brakingpressure according to the second distribution pattern. The ratio of thereduction of the front braking pressure with respect to the rear wheelbraking pressure may be constant. The amount of the reduction of thefront wheel braking pressure may be the same over the entire range inwhich the front wheel braking pressure varies, and under all runningconditions of the vehicle. Alternatively, the reduction amount of thefront wheel braking pressure may vary depending upon the vehicle runningcondition. For instance, the reduction amount of the front wheel brakingpressure may be increased with an increase in the friction coefficientof the road surface on which the vehicle runs.

The "predetermined value" of the front and rear braking pressures may beselected to substantially correspond to a threshold value of aproportioning valve above which the rear braking force is reduced withrespect to the front braking force during a minimum-load run of thevehicle. The "predetermined value" may be smaller than theabove-indicated value. Alternatively, the "predetermined value" may beselected to substantially correspond to a threshold value of theproportioning valve during a full-load run of the vehicle. The"predetermined value" may correspond to a value intermediate between thethreshold values of the proportioning valve during the minimum-load andfull-load runs of the vehicle.

As indicated above, the principle of the present invention can besatisfied if the distribution control device operates according to thefirst and second distribution patterns, which are formulated such thatthe rear wheel braking force defined by the second distribution patternis larger than that defined by the first distribution pattern in therange where the front and rear wheel braking forces are smaller than therespective predetermined values. Namely, the principle of the presentinvention requires the first and second distribution patterns to beformulated such that the rear wheel braking force according to thesecond distribution pattern is smaller than that according to the firstdistribution pattern when the vehicle is braked with the relativelysmall braking forces applied to the front and rear wheels. However, thefirst and second distribution patterns may be formulated such that therear wheel braking force defined by the second distribution pattern islarger than that defined by the first distribution pattern over theentire range of the front and rear braking forces. Namely, the first andsecond distribution patterns may be formulated such that the rear wheelbraking force according to the second distribution pattern is smallerthan that according to the first distribution pattern, irrespective ofthe levels of the braking forces to be applied to the front and rearwheels.

The present braking system is not limited to 4-wheel motor vehicles butalso to 2-wheel motor vehicles, and applicable equally to both passengerand industrial vehicles such as trucks.

In conventional braking systems that use a load-sensing proportioningvalve, the distribution line of the proportioning valve considerablydeviates from the ideal distribution curve in the direction that causesthe rear wheel braking force to be smaller than the ideal or optimumvalue, particularly when the vehicle running in the full-load conditionis braked with relatively small braking forces applied to the front andrear wheels, as indicated in the graph of FIG. 1. The braking forcesapplied to the front and rear wheels are relatively small during theinitial period of operation of the brake pedal, or during braking on aroad surface having a low friction coefficient. Therefore, theconventional braking system is incapable of increasing the rear wheelbraking force to a sufficiently high level even where the risk oflocking of the rear wheel is nonexistent. This drawback arises becausethe distribution line of the proportioning valve used in theconventional system is the same for both the minimum-load run and thefull-load run of the vehicle, as long as the breaking forces arerelatively small. In this respect, it is noted that the distributionlines for the minimum-load run and the full-load run as shown in FIG. 1coincide with each other at the braking force values approach zero.Further, the conventional braking system is adapted to prevent lockingof the rear wheel prior to the locking of the front wheel.

To solve the above-identified drawback, the present braking system isprovided with the distribution control device which operates accordingto the selection of one of two distribution patterns which areformulated such that the rear wheel braking force according to thesecond distribution pattern is larger than that according to the firstdistribution pattern, at least when the front and rear braking forcesare relatively small (i.e., smaller than the respective predeterminedvalues). For example, the first and second distribution patterns may bedetermined such that the first distribution pattern is close or similarto the ideal distribution curve for the minimum-load run of the vehiclewhile the second distribution pattern is close or similar to the idealdistribution curve for the full-load run of the vehicle. As describedabove, the minimum-load and full-load runs of the vehicle are the twoextreme running conditions of the vehicle in terms of the load actingthereon, which govern the formulation of, in the above case, the firstand second distribution patterns of the front and rear wheel brakingforces. These running conditions are effective extremes because thevehicle is usually braked under a load condition between those twoextreme load conditions.

In summary, the present invention wherein the distribution controldevice operates according to the two different distribution patternspermits a sufficient increase in the rear wheel braking force withoutlocking of the rear wheel, at least when the braking forces applied tothe front and rear wheels are relatively small (i.e., smaller than therespective predetermined values). Accordingly, the present brakingsystem is capable of reducing the required braking distance of thevehicle while avoiding prior locking of the rear wheel.

The first optional object indicated above may be achieved according to afirst preferred form of the present invention, wherein the brakingsystem further comprises a hydraulic pressure source 200 forpressurizing a working fluid, and the front and rear brakes 202, 204include a front wheel and a rear brake cylinder 206, 208, respectively,as shown in FIG. 7. The front and rear wheel brake cylinders 206, 208are supplied with the working fluid pressurized by the hydraulicpressure source 200. The distribution control device 212 comprises (a) acheck valve 214 disposed between the front wheel brake cylinder 206 andthe hydraulic pressure source 200, and (b) selective disabling means 216for selectively disabling the check valve 214. The check valve 214permits a flow of the fluid in a first direction from the hydraulicpressure source 200 toward the front wheel brake cylinder 206 after thepressure generated by the pressure source becomes higher than thepressure in the front wheel brake cylinder 206 by more than apredetermined difference, whereby the pressure of the fluid to beapplied to the front wheel brake cylinder 206 through the check valve214 is reduced with respect to the pressure generated by said hydraulicpressure source 200. The check valve 214 inhibits flow of the fluid in asecond direction opposite to the first direction. The selectivedisabling means 216 disables the check valve 214, as needed, to preventfunctioning of the check valve 214 to reduce the pressure to be appliedto the front wheel brake cylinder 206.

The selective disabling means 216 may include, for example, (a) aby-pass passage which is in parallel connection with the check valve 214and which by-passes the check valve 214, and (b) a shut-off valvedisposed in the by-pass passage. This shut-off valve is opened tosubstantially disable the check valve 214 to prevent its pressurereducing function when the distribution of the front and rear wheelbraking forces is controlled according to the first distributionpattern. The shut-off valve is closed to enable the check valve 214 toperform its pressure reducing function when the distribution iscontrolled according to the second distribution pattern. The shut-offvalve may be a solenoid-operated shut-off valve which is energized byapplication of an electric current, or a pilot-operated shut-off valvewhich is operated by application of an external pilot force such as aforce produced by a suitable displaceable member, which is displacedwith a change in the vehicle load. The displaceable member may be aload-sensing member of a load-sensing proportioning valve of alinkage-type used in the braking system to sense the vehicle load. Thecheck valve 214 may comprise a valve seat, a valving member, and biasingmeans in the form of a spring, which biases the valving member tothereby hold the valving member seated on the valve seat. In this case,the selective disabling means 216 may include (a) a valve opening memberfor moving the valving member apart from the valve seat against abiasing force of the spring, and (b) a control device for operating thevalve opening member to an operated position for moving the valvingmember apart from the valve seat when the distribution control deviceshould control the distribution of the front and rear wheel brakingforces according to the first distribution pattern, and for operatingthe valve opening member to a non-operated position to permit the valvemember to be seated on the valve seat by the biasing force of the springwhen the distribution control device should control the distributionaccording to the second distribution pattern.

The hydraulic pressure source 200 may principally consist of a mastercylinder which mechanically generates a hydraulic pressure commensuratewith an operating force acting on a brake operating member.Alternatively, the hydraulic pressure source 200 may be an electricallyoperated pressure source which electrically generates a hydraulicpressure commensurate with the operating force or operating valve of thebrake operating member. The electrically operated pressure source may beprincipally consist of a pump and a solenoid-operated pressure controlvalve which regulates the delivery pressure of the pump. Alternatively,the electrically operated pressure source may use an electric motor. Theelectric motor's rotary motion is converted by a ball screw into alinear piston motion that generates a hydraulic pressure. In this case,a controller controls a motor that regulates hydraulic pressuregenerated by the linear motion of the piston.

In the above-identified first preferred form of the invention whereinthe distribution control device comprises the check valve 214 and theselective disabling means 216, the pressure of the working fluid to beapplied to the front wheel brake cylinder 206 is reduced with respect tothe pressure generated by the pressure source 200 by an amountcorresponding to the predetermined level of the check valve 214, whilethe pressure as generated by the pressure source 200 is applied to therear wheel brake cylinders 200, unless the selective disabling meansdisables the check valve 214 to prevent its pressure reducing function.Thus, the distribution of the front and rear wheel braking forces iscontrolled according to the second distribution curve. Namely, the frontwheel braking pressure is controlled to be lower than the rear wheelbraking pressure by an amount corresponding to the predetermined levelat which the check valve is opened permitting the flow of the fluidtherethrough toward the front wheel brake cylinder. Consequently, thesecond distribution pattern is established by the distribution controldevice so as to control the rear wheel braking force to be larger thanthe front wheel braking force as long as the disabling means is placedin its non-operated position.

The braking system according to the above first preferred embodiment ofthe invention wherein the distribution control device uses the checkvalve 214 as described above can be easily constructed and is relativelyinexpensive.

The second optional object indicated above may be achieved according toa second preferred form of the invention, wherein the distributioncontrol device controls the distribution of the front and rear wheelbraking forces according to the first distribution pattern when a loadacting on the vehicle is smaller than a predetermined load value, andaccording to the second distribution pattern when the load is notsmaller than the predetermined load value.

The above second preferred form of the invention directly utilizes theknown relationship of the ideal distribution of the front and rear wheelbraking forces that vary as a function of the load currently acting onthe vehicle. The distribution control device uses the first distributionpattern when the vehicle load is relatively small, and uses the seconddistribution pattern when the vehicle load is relatively large, so thatthe rear wheel braking force is controlled to be larger when the vehicleload is relatively large than when it is relatively small.

In the above second preferred form of the invention, the actualdistribution of the front and rear wheel braking forces can always becontrolled in relation to vehicle load, and the required vehicle brakingdistance can be reduced even when the vehicle load is relatively large.The actual distributions of the front and rear wheel braking forcesduring the minimum-load and full-load runs of the vehicle areillustrated in the graphs of FIG. 10, in which BF and BR represent thefront and rear wheel braking forces, respectively.

In the present second preferred form of the invention, the firstdistribution pattern is formulated to more effectively avoid the priorlocking of the rear wheel than the second distribution pattern. Whilethe vehicle load is relatively small, the load acting on the rear wheelis also relatively small and is likely to have a locking tendency. Insuch small-load condition, however, the distribution control devicecontrols the distribution of the front and rear wheel braking forcesaccording to the first distribution pattern, so as to effectively avoidthe prior locking of the rear wheel, and effectively prevent undesirablereduction in the steering or directional stability of the vehicle due toreduction in the cornering force of the rear wheel, which would arisefrom the locking of the rear wheel. 0n the other hand, the seconddistribution pattern is formulated to more effectively reduce therequired vehicle braking distance, than the first distribution pattern.Since the second distribution pattern is used while the vehicle load isrelatively large, the rear wheel can effectively utilize the frictioncoefficient of the road surface, to reduce the required brakingdistance.

In an anti-lock braking system, the braking pressures are regulated inan anti-lock manner so as to prevent locking of the wheels, even if thenon-anti-lock control distribution pattern of the front and rear wheelbraking forces, which will be normally established without the anti-lockcontrol, deviates from the ideal distribution pattern which correspondsto the actual vehicle load. Therefore, the actual distribution patterncan accurately coincide with the ideal distribution patterncorresponding to the actual vehicle load, irrespective of whether thevehicle is in the minimum-load run or full-load run, even if the rearwheel braking force according to the non-anti-lock control distributionpattern is relatively large.

Based on the above finding, the braking system according to theprinciple of the present invention may be adapted to effect an anti-lockcontrol of the braking forces according to a third preferred form ofthis invention, which is suitable for achieving the third optionalobject indicated above. In this third preferred form of the invention,the braking system is an anti-lock braking system of a diagonal orX-crossing type for a four-wheel motor vehicle, as shown in a hydrauliccircuit diagram of FIG. 8. The present braking system has two pressureapplication sub-systems which are connected to respective two mutuallyindependent pressurizing chambers of a master cylinder 224, each of thetwo pressure application sub-systems including (a) a front brakecylinder passage 226 connecting a corresponding one of the twopressurizing chambers of the master cylinder and a front wheel brakecylinder 206 of the front brake, (b) a rear brake cylinder passage 228connecting the front brake cylinder passage 226 and a rear wheel brakecylinder 208 of the rear brake, (c) a master cylinder cut valve 230 inthe form of a normally-open shut-off valve disposed in a portion of thefront brake cylinder passage 226 between the master cylinder 224 and apoint of connection of the front and rear brake cylinder passages 226,228, (d) a reservoir passage 234 connected at one of opposite endsthereof to the rear brake cylinder passage 228, (e) a reservoir 232connected to the other end of the reservoir passage, (f) a pressurereducing valve 236 in the form of a normally-closed shut-off valvedisposed in the reservoir passage 234, (g) a pump passage 238 connectedat one of opposite ends thereof to the reservoir 232 and at the otherend to at least one of the front and rear brake cylinder passages 226,228, (h) a pump 240 disposed in the pump passage 238 for delivering aworking fluid from the reservoir to a portion of each pressureapplication sub-system, and (i) a controller 241 operable in ananti-lock pressure control mode for controlling the master cylinder cutvalve 230, the pressure reducing valve 236 and the pump to effect ananti-lock pressure control operation for controlling pressures of thefluid in the front and rear wheel brake cylinders 206, 208 in ananti-lock manner, and wherein the distribution control device comprisesthe controller 241, and a pressure reduction control device 242 disposedin a portion of each pressure application sub-system which is other thanthe portion between the master cylinder and the connection of the frontand rear brake cylinder passages. The pressure reduction control deviceis adapted to apply to the front wheel brake cylinder 206 the pressureas generated by a first hydraulic pressure source in the form of themaster cylinder 224, to thereby establish the first distributionpattern, when the controller 241 is not placed in the anti-lock pressurecontrol mode. The controller 241 and the pressure reduction controldevice 242 cooperate to establish the second distribution pattern suchthat the pressure generated by a second hydraulic pressure source whichconsists of at least one of the master cylinder 224 and the pump 240 isreduced by the pressure reduction control device 242 and is then appliedto the front wheel brake cylinder 208, when the controller 241 is placedin the anti-lock pressure control mode.

The anti-lock braking system constructed as described above may bedesirably provided with a proportioning valve adapted to reduce thepressure in the rear wheel brake cylinder 208 with respect to thepressure in the front wheel brake cylinder 205 after the pressuregenerated by the hydraulic source (master cylinder or pump) becomeshigher than the front wheel brake cylinder 205 by more than apredetermined difference, which may be either fixed or variable. In thisinstance, the proportioning valve may be disposed in a portion of therear brake cylinder passage 228 between the rear wheel brake cylinder208 and the point of connection of the rear brake cylinder passage 228and the pump passage 238. In this case, means are provided to disablethe proportioning valve during an operation of the pump 240 to increasethe rear wheel brake cylinder pressure. This disabling means preventsthe proportioning valve from operating to reduce the rear wheel brakecylinder pressure while increasing the rear wheel brake cylinderpressure by operation of the pump 240.

In the anti-lock braking system illustrated in FIG. 8, the pump 240functions as the hydraulic pressure source during the anti-lock pressurecontrol of the front and rear wheel brake cylinders 206, 208. However,the master cylinder 224 may be used as the hydraulic pressure sourcewhen the pump 240 is required to increase the front and rear wheel brakecylinder pressures while no working or brake fluid is stored in thereservoir 232. In this exceptional case, the master cylinder cut valve230 is opened, to permit the fluid pressurized by the master cylinder224 to be supplied to the front and rear wheel brake cylinders 206, 208to thereby increase the pressures in these brake cylinders. In thepresent third preferred form of the invention, therefore, at least onemaster cylinder 224 and pump 240 are used as the hydraulic pressuresource during the anti-lock pressure control of the front and rear wheelbrake cylinders 206, 208.

Although the braking system of FIG. 8 is adapted such that the deliveryor output end of the pump passage 238 is connected to the rear brakecylinder passage 228, which is downstream of the master cylinder cutvalve 230, the output end of the pump passage 238 may be connected to aportion of the front brake cylinder passage 226 which is upstream ordownstream of the master cylinder cut valve 230.

While the pressure reduction control device 242 is disposed in the rearbrake cylinder passage 228 in the braking system of FIG. 8, the pressurereduction control device 242 may be disposed in the front brake cylinderpassage 226.

In the braking system of FIG. 8, a by-pass passage is provided toby-pass the master cylinder cut valve 230, and a check valve 250 isdisposed in this by-pass passage, so as to inhibit a flow of the fluidfrom the master cylinder 224 toward the front and rear wheel brakecylinders 206, 208. The check valve 250 permits fluid to flow in theopposite direction, with the valve opening pressure difference beingsubstantially zero. This check valve 250 functions not only as a valvefor permitting a rapid return of the fluid to the master cylinder 224upon releasing of the brake operating member, but also as a releasevalve for preventing an excessive rise of the pressure in the frontwheel brake cylinder 206 above the pressure level of the master cylinder200 during an operation of the pump 240 to increase the front wheelbrake cylinder pressure. Reference numeral 252 also denotes a checkvalve for the rear wheel brake cylinder 208. The check valve 252 has thesame function as the check valve 250 for the front wheel brake cylinder206.

In the anti-lock braking system of the diagonal or X-crossing typehaving the two pressure application sub-systems according to the thirdpreferred form of the invention, each pressure application sub-system isbasically designed to effect simultaneous control of the front and rearwheel braking pressures. During normal operation of the system withoutanti-lock pressure control, the pressure as generated by the pressuresource is applied to the front wheel brake cylinder, whereby the frontand rear wheel braking forces are controlled according to the firstdistribution pattern. During the anti-lock pressure control, thepressure generated by the pressure source is reduced by the pressurereduction control device before it is applied to the front wheel brakecylinder. Thus, the controller and the pressure reduction control devicecooperate to establish the second distribution pattern so that thepressure in the front wheel brake cylinder is higher than that in therear wheel brake cylinder.

The anti-lock braking system according to the above third preferred formof the invention has an advantage as described below by reference to thegraphs of FIG. 11.

While the present anti-lock braking system is in the normal pressurecontrol mode, the distribution of the front and rear wheel brakingforces is controlled according to the first distribution pattern,regardless of whether the vehicle is in the minimum-load run or in thefull-load run. As a result, the actual distribution pattern or curveduring the full-load run deviates from the ideal distribution curve suchthat the rear wheel braking force BR is smaller than the optimum orideal value, as indicated at right in FIG. 11(a).

While the braking system is in the anti-lock pressure control mode, theactual distribution curve established as a result of the anti-lockpressure control is sufficiently close and similar to the idealdistribution curve during the minimum-load run, even if thenon-anti-lock control distribution curve deviates from the idealdistribution curve such that the rear wheel braking force BR is largerthan the ideal value. During the full-load run, on the other hand, thenon-anti-lock control distribution curve is sufficiently close andsimilar to the ideal distribution curve for the full-load run, namely,does not deviates from the ideal distribution curve such that the rearwheel braking force BR is larger than the ideal value. Accordingly, theactual distribution curve is sufficiently close and similar to thenon-anti-lock control distribution curve. Thus, the actual distributioncurve substantially coincides with the ideal distribution curve for thefull-load run, as indicated in FIG. 11(b), irrespective of theminimum-load run or full-load run of the vehicle, owing to the anti-lockpressure control operation during the minimum-load run, and owing to thenon-anti-lock control distribution which is utilized during thefull-load run.

In the present third preferred form of the invention, the firstdistribution pattern is established in the normal pressure control modeirrespective of the minimum-load or full-load run of the vehicle, andduring the minimum-load run in the anti-lock pressure control mode,while the second distribution pattern is established during thefull-load run in the anti-lock pressure control mode.

The present third preferred form may be suitably combined with thesecond preferred form indicated above. In this instance, the firstdistribution pattern is established during the minimum-load run in boththe normal pressure control mode and the anti-lock pressure controlmode, while the second distribution pattern is established during thefull-load run in both of the normal and anti-lock pressure controlmodes.

It will be understood from the above description that the anti-lockbraking system according to the third preferred form of the inventionassures the actual distribution pattern of the front and rear wheelbraking forces which is as close as possible to the ideal distributioncurve or pattern in the anti-lock pressure control mode, whereby therequired braking distance of the vehicle in the anti-lock pressurecontrol mode can be significantly reduced.

The fourth optional object indicated above may be achieved according toa fourth preferred form of the present invention, wherein the brakingsystem is an anti-lock braking system of a diagonal or X-crossing typefor a four-wheel motor vehicle, as illustrated in FIG. 9. Each pressureapplication sub-system has two pressure application sub-systems whichare connected respectively to two mutually independent pressurizingchambers of a master cylinder 224, each of the two pressure applicationsub-systems including (a) a front brake cylinder passage 226 connectinga corresponding one of the two pressurizing chambers of the mastercylinder 224 and a front wheel brake cylinder 206 of the front brake,(b) a rear brake cylinder passage 228 connecting the front brakecylinder passage 226 and a rear wheel brake cylinder 208 of the rearbrake, (c) a master cylinder cut valve 230 in the form of anormally-open shut-off valve disposed in a portion of the front brakecylinder passage 226 between the master cylinder 224 and a point ofconnection of the front and rear brake cylinder passages 226, 228, themaster cylinder cut valve 230 being closed when the braking system is inan anti-lock pressure control mode, and opened when the braking systemis not in the anti-lock pressure control mode, (d) an intermediate valve254 in the form of a normally-open shut-off valve disposed in the rearbrake cylinder passage 228, (e) a reservoir passage 234 connected at oneof opposite ends thereof to a portion of the rear brake cylinder passage228 between the intermediate valve 254 and the rear wheel brake cylinder228, (f) a reservoir 232 connected to the other end of the reservoirpassage 234, (g) a pressure reducing valve 236 in the form of anormally-closed shut-off valve disposed in the reservoir passage 234,(h) a pump passage 238 connected at one of opposite ends thereof to thereservoir 232 and at the other end to a portion of the rear brakecylinder passage 228 between the intermediate valve 254 and a point ofconnection of the front and rear brake cylinder passages 226, 228, (i) apump 240 disposed in the pump passage 238 for delivering a working fluidfrom the reservoir 232 to a portion of the each pressure applicationsub-system, and (j) a controller 241 operable in the anti-lock pressurecontrol mode for controlling the master cylinder cut valve 230, theintermediate valve 254, the pressure reducing valve 236 and the pump 240to effect an anti-lock pressure control operation for controllingpressures of the fluid in the front and rear wheel brake cylinders 206,208 in an anti-lock manner, and wherein the distribution control devicecomprises the controller 241, and a check valve device 256 disposed in aportion of the rear brake cylinder passage 228 between the point ofconnection of the front and rear brake cylinder passages 226, 228 and apoint of connection of the rear brake cylinder passage 228 and the pumppassage 238, the check valve device comprising a first check valve 258,and a second check valve 260, the first check valve permitting a flow ofthe fluid therethrough in a first direction from the pump 240 toward thefront wheel brake cylinder 206 after the pressure generated by the pumpbecomes higher than the pressure in the front wheel brake cylinder 206by more than a predetermined difference, and inhibiting a flow of thefluid therethrough in a second direction opposite to the firstdirection, the second check valve 260 permitting a flow of the fluidtherethrough in the second direction and inhibiting a flow of the fluidtherethrough in the first direction.

The check valve device used in the fourth preferred form of theinvention can be considered one example of the pressure reductioncontrol device.

In the above form of the anti-lock braking system, the master cylindercut valve 230, pressure reducing valve 236 and intermediate valve 254are provided to control the pressures in the front and rear wheel brakecylinders. As shown in FIG. 9, the intermediate valve 254 is disposedbetween the front and rear wheel brake cylinders 206, 208, to partiallyintroduce independent pressure control of the front and rear wheel brakecylinder pressures into the basic arrangement to simultaneously controlthese brake cylinder pressures. In the absence of the intermediate valve254, the front and rear wheel brake cylinder pressures would always becontrolled in the same pressure control manner or mode, and thesepressures would have the same relationship. In the presence of theintermediate valve 254, the front and rear wheel brake cylinderpressures can be regulated independently of each other, depending uponthe locking tendencies of the corresponding front and rear wheels. Forexample, the front wheel brake cylinder pressure can be increased whilethe rear wheel brake cylinder pressure can be reduced.

The present anti-lock braking system is adapted such that the output endof the pump passage 238 is connected to a portion of the rear brakecylinder passage 228, which is upstream of the intermediate valve 254.In this arrangement, the front wheel brake cylinder pressure isincreased while the rear wheel brake cylinder pressure is reduced orheld constant, when the pump 240 is operated to deliver the pressurizedfluid while the intermediate valve 254 is held closed. Thus, the frontwheel brake cylinder pressure can be increased independently of the rearwheel brake cylinder pressure. Therefore, this arrangement is effectiveto reduce the required braking distance where the vehicle is braked whenthe front and rear wheels lie on respective areas of the road surfacewhich have relatively high and low friction coefficient values,respectively. Namely, the present arrangement makes it possible tomaximize the front wheel brake cylinder pressure to take advantage ofthe relatively high friction coefficient of the road surface areaunderneath the front wheel.

In the above anti-lock braking system of FIG. 9, the check valve device256 of the distribution control device is disposed in the portion of therear brake cylinder passage 228 between the points of connection to thefront brake cylinder passage 226 and the pump passage 238. Further, thecheck valve device 256 comprises the first and second check valves 258,260 which are arranged in parallel to each other, such that the firstcheck 258 permits the fluid flow in the first direction from the pump240 toward the front wheel brake cylinder 206 only after the deliverypressure of the pump 240 becomes higher than the pressure in the frontwheel brake cylinder 206 by more than the predetermined difference,while the second check valve 260 permits the fluid flow in the seconddirection opposite to the first direction, with the valve openingpressure difference being substantially zero.

While the braking system of FIG. 9, or its controller 241, is not placedin the anti-lock pressure control mode (i.e., placed in the normalpressure control mode) the fluid as pressurized by the master cylinder224 is supplied to the front wheel brake cylinder 206 through the openmaster cylinder cut valve 230, and to the rear wheel brake cylinder 208through the second check valve 260 and the normally-open intermediatevalve 254, without pressure reduction by the check valve device 256.Thus, the pressure applied to the rear wheel brake cylinder 208 is notreduced with respect to the master cylinder pressure, whereby thedistribution of the front and rear wheel braking forces is controlledaccording to the first distribution pattern which causes the front andrear wheel brake cylinder pressures to be equal to each other.

While the controller 241 is placed in the anti-lock pressure controlmode, the master cylinder cut valve 230 is closed and the pump 240 isoperated, so that the fluid delivered from the pump 240 is applied tothe front wheel brake cylinder 206 through the first check valve 258.Since the fluid is permitted to flow through the first check valve 258in the first direction only after the delivery pressure of the pump 240has been raised to the predetermined valve opening threshold level, thepressure to be applied to the front wheel brake cylinder 206 is reducedto the delivery pressure of the pump 240 by an amount corresponding tothe opening pressure difference of the first check valve 258. However,the fluid as delivered from the pump 240, is applied to the rear wheelbrake cylinder 208, without pressure reduction by the first check valve258. Accordingly, the pressure in the front wheel brake cylinder 206 iscontrolled to be lower than that in the rear wheel brake cylinder 208 bythe amount corresponding to the opening pressure difference of the firstcheck valve 258, while the front and rear wheel brake cylinder pressuresare increased in the anti-lock pressure control mode with the pump 240being operated. Therefore, the distribution of the front and rear wheelbraking forces in the anti-lock pressure control mode is controlledaccording to the second distribution pattern, which is substantially thesame as the non-anti-lock control distribution pattern which isestablished in the normal pressure control mode (i.e., the pattern thatcauses the rear wheel braking force to be relatively high).

The distribution control device constructed according to the fourthpreferred form of the invention may also be used in the third preferredform of the invention described above.

In the anti-lock braking system of the X-crossing type according to thepresent fourth preferred form of the invention, the intermediate valvepermits the front and rear wheel brake cylinder pressures to becontrolled independently of each other in each pressure applicationsub-system, which is basically designed for simultaneous control of thefront and rear wheel brake cylinder pressures. Thus, the intermediatevalve permits a greater amount of flexibility to control the anti-lockpressures of front and rear wheel brake cylinders as running conditionof the vehicle.

The fifth optional object indicated above can be achieved according toone advantageous arrangement of the above fourth preferred form of thepresent invention which includes the check valve device. In thisarrangement, at least a part of the portion of the rear brake cylinderpassage 228 between the point of connection of the front and rear brakecylinder passages 226, 228 and the point of connection of the rear brakecylinder passage 228 and the pump passage 238 consists of a first and asecond passage which are concentric with and mutually independent ofeach other and which have a circular and an annular cross sectionalshape, respectively. The first check valve 256 is disposed in one ofthese first and second passages while the second check valve 260 isdisposed in the other of the first and second passages.

The first check valve 258 may include a valving member in the form of aball which is seated on a valve seat under a biasing action of suitablebiasing means. The second check valve 260 may be a valve which includesan annular one-way sealing member made of an elastic material.

In the above advantageous arrangement, the first and second check valves258, 260 are disposed in the one and the other of the first and secondpassages which have the circular and annular cross sectional shapes andwhich are disposed concentrically with each other such that one of thecircular and annular passages is located within the other passage.Namely, the circular and annular passages are not arranged in parallelin the diametric direction thereof. This arrangement is effective tominimize the maximum dimension of the check valve device 256 in thediametric direction of those first circular and second annular passages,whereby the overall size of the check valve device 256 can beeffectively reduced. Accordingly, the braking system incorporating thecheck valve device can be manufactured without a considerable increasein required installation space.

The sixth optional object indicated above may be achieved according toanother advantageous arrangement of the above fourth preferred form ofthis invention, wherein the controller 241 has a plurality of pressurecontrol modes which are selectively established to control the mastercylinder cut valve 230, the intermediate valve 254 and the pressurereducing valve 236 in the anti-lock manner. The pressure control modesinclude (1) a mode in which the intermediate valve and the pressurereducing valve are both open while the master cylinder cut valve isclosed, to reduce the pressures in both of the front and rear wheelbrake cylinders 206, 208, (2) a mode in which the master cylinder cutvalve and the intermediate valve are both closed while the reducingvalve is open, to increase the pressure in the front wheel brakecylinder and reduce the pressure in the rear wheel brake cylinder, byoperation of the pump 240, and (3) a duty-cycle pressure control mode inwhich the master cylinder cut valve and the pressure reducing valves areboth closed while the intermediate valve is alternately closed andopened to increase the pressures in the front and rear wheel brakecylinders by operation of the pump 240.

In the above advantageous arrangement, the controller 241 has theduty-cycle control mode in which the intermediate valve 254 isalternately closed and opened to increase the pressures in the front andrear wheel brake cylinders by operation of the pump 240. In theanti-lock pressure control mode, the fluid delivered from the pump 240is supplied to only the front wheel brake cylinder 206 while theintermediate valve 254 is in the closed state, and is supplied to notonly the front wheel brake cylinder 206 but also the rear wheel brakecylinder 208 while the intermediate valve 254 is in the open state.Accordingly, the repetition of the alternate closing and opening of theintermediate valve 254 makes it possible to increase the pressure ineach of the front and rear wheel brake cylinders at a rate which isintermediate between a rate when the intermediate valve is kept open anda rate when the intermediate valve is kept closed. The duty-cyclecontrol mode improves the freedom of control of the front and rear wheelbraking pressures.

The seventh optional object indicated above may be achieved if thecontroller 241 is adapted to comprise means for changing a duty cycle ofthe intermediate valve 254 in the duty-cycle pressure control mode.Although the duty cycle may be fixed, it is desirable to vary the dutycycle as a function of the specific running condition of the vehicle.This allows the braking pressures to be controlled with a higher degreeof freedom and thereby enhances braking performance. This arrangementpermits the front and rear wheel braking pressures to be increased atoptimum rates with the duty cycle of the intermediate valve beingsuitably controlled in the anti-lock pressure control mode, which is afunction of the running condition of the vehicle.

The duty cycle of the intermediate valve 254 may be changed by modifyingthe ratio of the time period when the intermediate valve is open to thetime period when the intermediate valve is closed.

The rates of increase of the wheel braking pressures may be changed bysome mechanical means, for instance, flow restrictions such as orifices,which are disposed in the front and rear brake cylinder passages andwhich possess variable cross sectional areas of fluid flow. In thedesirable arrangement indicated above, such mechanical means arereplaced by the means for changing the duty cycle of the intermediatevalve as described above, said means are constituted by a controlprogram or electronic circuitry provided in the controller. Thus, thepresent arrangement permits the rates of increase of the brake cylinderpressures while avoiding an increase in the cost of the braking system.

Generally, individual vehicles have different optimum increase rates ofthe wheel brake cylinder pressures. Optimum increase rates assureadequate anti-lock control of the wheel braking forces. These optimumincrease rates depend on the specific characteristics of the brakingsystem, such as the ratio of the diameters of the front and rear wheelbrake cylinders, and to the specific braking conditions of the vehicle,such as the actual braking effect and load distribution of the vehicleon the front and rear wheels. In the above arrangement, the rate ofpressure increase of the front and rear wheel brake cylinders can beeasily controlled without a costly mechanism. In doing so the duty cycleof the intermediate valve is controlled by the controller, which meetsthe specific characteristics of the braking system of the particularvehicle.

The eighth optional object indicated above may be achieved if theabove-indicated means for changing the duty cycle of the intermediatevalve is adapted to change the duty cycle on the basis of either the ofpressure reducing tendency of the front wheel brake cylinder or thepressure reducing tendency of the rear wheel brake cylinder, or both.These tendencies are observed during control of the anti-lock pressuresby said controller. For example, the pressure reducing tendencies, orhystereses, are expressed by the number or frequency of observedpressure reductions of the front and rear wheel brake cylinders, thepressure reducing time periods of these cylinders, or the rates at whichthe pressure have been reduced.

Such previous and present pressure reducing tendencies directly reflectthe locking tendencies of the front and rear wheels. Accordingly, thelocking tendencies of the wheels can be detected by monitoring thepressure reducing tendencies in the anti-lock pressure control mode. Ifthe pressure in the rear wheel brake cylinder has been reduced morefrequently than that in the front wheel brake cylinder, it is possibleto determine that the rear wheel has a higher locking tendency that thefront wheel. In this case, it is desirable to reduce the rear brakecylinder pressure and increase the front wheel brake cylinder pressure.In other words, it is desirable to determine the duty cycle of theintermediate valve, so as to establish a distribution of the pressuresof the front and rear wheel braking cylinders which causes a higher rateof increase in the pressure of the front wheel brake cylinder and alower rate of increase in the rear wheel brake cylinder when the rearwheel brake cylinder has exhibited a higher tendency of pressurereduction that the front wheel brake cylinder.

In the above arrangement in which the duty cycle is changed on the basisof the pressure reducing tendencies of the front and rear wheel brakecylinders, the braking capacities of the wheels can be maximized withthe locking tendency of each wheel taken into consideration. In thisrespect, it is noted that the locking tendencies of the wheels aredependent on various factors such as the friction coefficients of theroad surface areas beneath the wheels, and from the wheel brakingtorques and loads acting on the wheels. Therefore, the rates of increaseof the brake cylinder pressures which are determined by the controlledduty cycle of the intermediate valve can be adequately controlled withhigh precision more accurately to reflect the actual braking or lockingtendencies of the wheels.

The pressure reducing tendencies of the front and rear wheel brakecylinders can be detected by monitoring the signals generated toenergize or de-energize the solenoids of the master cylinder cut-offvalve, intermediate valve and pressure reducing valve. Thus, the abovearrangement does not require an exclusive sensor for detecting thepressure reducing tendencies of the wheel brake cylinders, and istherefore available at a relatively low cost while it is capable ofadequately controlling the duty cycle of the intermediate valve, namely,the pressure increase rates of the wheel brake cylinders.

The means for changing the duty cycle of the intermediate valve may beadapted to vary the duty cycle as a function of an amount of shift of avehicle load in a running direction of the vehicle. The load shift maybe detected on the basis of the deceleration of the vehicle in therunning direction. Specifically, upon braking of the vehicle, the loadacting on the front wheels increases while the load acting on the rearwheels decreases because the vehicle load in the vehicle runningdirection shifts. This means that the front wheel brake cylinderpressure should be increased to increase the braking force on the frontwheel while the rear wheel brake cylinder pressure should be reduced toprevent the rear wheel from locking. To this end, it is preferable todetermine the duty cycle of the intermediate valve, so as to establish adistribution of the pressures of the front and rear wheel brakecylinders which causes a higher rate of increase in the pressure of thefront wheel brake cylinder and a lower rate of increase in the rearwheel brake cylinder when the amount of shift of the load to a frontwheel for which the front wheel brake cylinder is provided is relativelylarge than when the amount of shift of the load to the front wheel isrelative small.

Alternatively, the means for changing the duty cycle of the intermediatevalve may be adapted to vary the duty cycle as a function of an amountof shift of a vehicle load in the lateral direction of the vehicle,which may be detected on the basis of the lateral acceleration of thevehicle. In pressure application sub-systems, in which the front andrear wheels are braked by the front and rear wheel brake cylinders arerespectively located on the outer and inner sides of the vehicle turningline along which the vehicle is turning, the load acting on the frontwheel increases while the load acting on the rear wheel decreases, dueto a shift of the vehicle load in the lateral direction (i.e.,perpendicular to the running direction). In this case as well, it isdesirable to increase the front wheel brake cylinder pressure toincrease the braking force of the front wheel and decrease the rearwheel brake cylinder pressure to prevent the locking of the rear wheel.In this respect, it is preferable to determine the duty cycle of theintermediate valve, to establish a distribution of the pressures of thefront and rear wheel brake cylinders. This preferable duty cycle causesa higher rate of increase in the pressure in the front wheel brakecylinder and a lower rate of increase in the rear wheel brake cylinderwhen the amount of shift of the load is relatively large than when theamount of shift of the load is relatively small. This results in thepressure application sub-system in which the front and rear wheels arelocated on the outer and inner sides of the vehicle turning linerespectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-identified optional objects, features, and advantages of thepresent invention will be better understood by reading the followingdetailed description of the preferred embodiments of the presentinvention in combination with the accompanying drawings, in which:

FIG. 1 is a graph explaining a relationship between braking forces offront and rear wheels in a known braking system of a motor vehicle;

FIG. 2 is a schematic view of one example of a known anti-lock brakingsystem of diagonal or X-crossing type;

FIG. 3 is a schematic view of another example of a known anti-lockbraking system of the same type as that of FIG. 2;

FIG. 4 is a schematic view of one example of a known anti-lock brakingsystem of independent front-rear braking force control type;

FIG. 5 is a schematic view of another example of the known anti-lockbraking system of the same type as that of FIG. 4;

FIG. 6 is a block diagram illustrating a principle of the presentinvention;

FIG. 7 is a block diagram illustrating one preferred form of the presentinvention;

FIG. 8 is a hydraulic circuit diagram schematically showing a furtherpreferred form of this invention;

FIG. 9 is a hydraulic circuit diagram schematically showing a stillfurther preferred form of the invention;

FIG. 10 is a graph explaining an advantage of the preferred form of theinvention of FIG. 7;

FIGS. 11a and 11b are graphs explaining an advantage of the preferredform of the invention of FIG. 8;

FIG. 12 is a schematic view of one embodiment of the braking system ofthe present invention;

FIG. 13 is a fragmentary front elevational view in cross section of aproportioning valve provided in the braking system of FIG. 12;

FIG. 14 is a graph indicating a pressure control characteristic of acheck valve device provided in the braking system of FIG. 12;

FIG. 15 is a flow chart illustrating a pressure reduction controlroutine executed by a computer of a controller used in the brakingsystem of FIG. 12;

FIG. 16 is a graph explaining a relationship between braking forces offront and rear wheels as controlled in the braking system of FIG. 12;

FIG. 17 is a schematic view of an anti-lock braking system according toanother embodiment of this invention;

FIG. 18 is a front elevational view in cross section of a proportioningvalve provided in the braking system of FIG. 17;

FIG. 19 is a hydraulic circuit diagram schematically showing flows of abrake fluid to and from a master cylinder, a pump and front and rearwheel brake cylinders in the braking system of FIG. 17;

FIG. 20 is a graph illustrates the changes in the front and rear wheelbrake cylinder pressures as controlled differently in an anti-lockfashion in fourth and fifth modes of operation of the braking system ofFIG. 17;

FIG. 21 is a graph of a relationship between the braking forces of thefront and rear wheels as controlled in the braking system of FIG. 17;

FIG. 22 is a front elevational cross-section view showing details ofconstruction of a check valve device and a second shut-off valve whichare provided in the embodiment of FIG. 17;

FIG. 23 is a flow chart illustrating a routine of a computer controllerused in the braking system of FIG. 17, and controls the second shut-offvalve;

FIG. 24 is a flow chart illustrating a sub-routine executed in step $40of the routine of FIG. 23;

FIG. 25 is a flow chart illustrating a routine executed by the computerfor determining an OFF time T₁ of the solenoid of the second shut-offvalve;

FIG. 26 is a graph explaining a relationship between intermittent fluiddelivery of a pump used in the braking system of FIG. 17 andenergization and de-energization of the solenoid of the second shut-offvalve;

FIG. 27 is a schematic view showing an anti-lock braking systemaccording to a further embodiment of the present invention;

FIG. 28 is a front elevational cross-section of a proportioning valveused in the braking system of FIG. 27;

FIG. 29 is a graph detailing the relationship between the braking forcesof the front and rear wheels as controlled in the braking system of FIG.27; and

FIG. 30 is a schematic view showing a yet further embodiment of theanti-lock braking system of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 12, there will be described one embodiment ofthe present invention for a braking system of the diagonal or X-crossingtype motor vehicle's.

In FIG. 12, reference numeral 10 denotes a master cylinder whichfunctions as a hydraulic pressure source. The master cylinder 10 is of atandem type in which two mutually independent fluid pressurizingchambers are disposed in series. The master cylinder 10 is linked with abrake operating member in the form of a brake pedal 14 through a booster12. Upon operation or depression of the brake pedal 14 by the driver oroperator of the motor vehicle, equal fluid pressures are mechanicallygenerated in the two pressurizing chambers of the master cylinder 10.

One of the pressurizing chambers of the master cylinder 10 is connectedto brake cylinders of hydraulically operated brakes for a front leftwheel and a rear right wheel of the vehicle, while the otherpressurizing chamber is connected to brake cylinders of hydraulicallyoperated brakes for a front right wheel and a rear left wheel of thevehicle. These brake cylinders are hereinafter referred to as "wheelbrake cylinders." Thus, the braking system has two mutually independentpressure application sub-systems one of which has the front left wheelbrake cylinder and the rear right wheel brake cylinder, and the other ofwhich has the front right wheel brake cylinder and the rear left wheelbrake cylinder. Since the two pressure application sub-systems areidentical in construction with each other, only one of these sub-systemsis illustrated in FIG. 1 and will be described below.

In each pressure application sub-system, the corresponding pressurizingchamber of the master cylinder 10 is connected to the front wheel brakecylinder 22 through a front brake cylinder passage 20. A rear brakecylinder passage 24 is connected at one end thereof to the front wheelbrake cylinder passage 20 and at the other end to the rear wheel brakecylinder 30.

In the rear brake cylinder passage 24, there is provided a proportioningvalve 34 (hereinafter referred to as "P valve 34"). The P valve 34 is apressure reducing valve which operates so that the fluid pressure asgenerated by the master cylinder 10 (hereinafter referred to as "mastercylinder pressure") is applied to the rear wheel brake cylinder 30without reduction of the master cylinder pressure, until the mastercylinder pressure reaches a predetermined threshold level, and so thatthe master cylinder pressure higher than the threshold level is reducedat a predetermined ratio, so that the reduced pressure is applied as thebraking pressure to the rear wheel brake cylinder 30. This P valve 40 isa load-sensing proportioning valve in which the threshold levelindicated above increases with an increase in the amount of load whichacts on the vehicle. The construction of the load-sensing P valve 40will be described in detail by reference to FIG. 13.

The P valve 34 is mounted on the vehicle such that the P valve 34 isconnected to the portions of sprung and unsprung members of the vehiclesaid portions correspond to the rear wheels. The P valve 34 has a valvebody 40 fixed to the vehicle body, which is a sprung member of thevehicle. In the valve body 40, there is fluid-tight and slidablyreceivable stepped piston 42. With the piston 42 received in the valvebody 40, the space formed in the valve body 40 is divided into an inputchamber 44 communicating with the master cylinder 10, and an outputchamber 46 communicating with the rear wheel brake cylinder 30. Theseinput and output chambers 44, 46 are normally held in communication witheach other through a communication passage 48 formed through the piston42. A shut-off valve 50 is disposed in the communication passage 48. Theshut-off valve 50 has a valving member in the form of a ball 52, a valveseat 54, and biasing means in the form of a return spring 56 for biasingthe ball 52 in a direction toward the valve seat 54. Normally, theshut-off valve 50 is held open with the ball 52 spaced apart from thevalve seat 54 by a stationary valve opening member 60. When the mastercylinder pressure, that is, the pressure in the input chamber 44 exceedsthe threshold level of the P valve 34, the piston 42 is moved in adirection away from the valve opening member 60, and the ball 52 iseventually seated on the valve seat 54 under a biasing force of thereturn spring 56, whereby the communication passage 48 is closed. As aresult, the input and output chambers 44, 46 are disconnected from eachother. Thus, the pressure in the output chamber 44 is reduced withrespect to that in the input chamber 44, at a ratio determined by theratio of the pressure-receiving areas of the piston 42.

One of the end portions of the piston 42 on the side of the inputchamber 44 extends through the valve body 40 and is exposed to theatmosphere, so that the end face of the exposed end portion of thepiston 42 is held in abutting contact with a lever 64 which is pivotallyconnected at its proximal end to the valve body 40. The lever 64 isbifurcated at an intermediate portion, and has two distal end portions.One of these distal end portions is connected to the valve body 40through biasing means in the form of an adjusting spring 66, while theother distal end portion is connected to the unsprung member (e.g., rearaxle) of the vehicle through a main spring 68. This main spring 68functions as force generating means which generates a force that varieswith the amount of relative displacement between the portions of thesprung and unsprung members of the vehicle, which portions correspond tothe rear wheels. According to this arrangement, the lever 64 applies tothe piston 42 a force which corresponds to a difference obtained bysubtracting the biasing force of the main spring 68 from the biasingforce of the adjusting spring 66. During the full-load run of thevehicle, the biasing force of the main spring 68 is smaller than duringthe minimum-load run, as a result of contraction of the main spring 68,whereby the force F which is transferred to the piston 42 through thelever 64 is increased, and the force which the ball 52 should overcomefor opening the shut-off valve 50 is accordingly increased. Therefore,the threshold level of the master cylinder pressure at which the P valve34 begins to reduce the braking pressure in the rear wheel cylinder 30is raised as the load acting on the vehicle (i.e., the rear wheels)increases.

In a portion of the front brake cylinder passage 22 between the frontwheel brake cylinder 20 and the point of connection of the front andrear brake cylinder passages 22, 24, there is disposed a check valvedevice 70 having a first check valve 72 and a second check valve 74which are disposed in parallel. The directions in which the first andsecond check valves 72, 74 permit flow of the brake fluid, respectively,are opposite to each other.

The first check valve 72 permits a flow of the brake fluid in thedirection from the master cylinder 10 toward the front wheel brakecylinder 20. Namely, the brake fluid is permitted to flow through thefirst check valve 72 in the above-identified direction when the pressureof the master cylinder 10 is higher than the pressure in the front wheelbrake cylinder 20 by more than a predetermined difference (valve openingpressure difference), which is not substantially zero. That is, thefirst check valve 72 permits the fluid flow in the above-identifieddirection after the pressure on the upstream side of the check valve 72becomes higher than the pressure in the front wheel brake cylinder 20 bymore than the predetermined valve opening pressure difference. The firstcheck valve 72 inhibits a flow of the brake fluid in the direction fromthe front wheel brake cylinder 20 toward the master cylinder 10. Theopening pressure difference of the first check valve 72 is determined,for example, by a biasing force of a spring which biases a valvingmember in the form of a ball used in the first check valve 72. It isnoted that the purpose of the first check valve 72 is not to inhibit theflow of the brake fluid in the above-identified direction but to reducethe braking pressure in the front wheel brake cylinder 20, the firstcheck valve 72 can be considered as a pressure reducing valve of a checkvalve type.

On the other hand, the second check valve 74 permits a flow of the brakefluid in the direction from the front wheel brake cylinder 20 toward themaster cylinder 10, with the valve opening pressure difference beingsubstantially zero. Since the direction of flow of the brake fluidthrough the second check valve 74 is opposite to that of the flow of thebrake fluid through the first check valve 72, these two check valves 72,74 cooperate to permit bidirectional flow of the brake fluid through thecheck valve device 70.

The check valve device 70 thus constructed provides a hysteresis in therelationship between the master cylinder pressure and the front wheelbraking pressure (pressure in the front wheel brake cylinder 20). Thishysteresis will be described by reference to the graph of FIG. 14.

When the master cylinder pressure is increased from zero by depressionof the brake pedal 14 by the vehicle operator, the first check valve 72is held closed until the master cylinder pressure becomes higher thanthe pressure in the front wheel brake cylinder 20 by more than thepredetermined valve opening pressure difference. Therefore, the brakefluid is not permitted to flow from the master cylinder 10 toward thefront wheel brake cylinder 20, and the front wheel braking pressure isheld at zero, until the preset opening pressure difference of the firstcheck valve 72 is exceeded. Accordingly, only the master cylinderpressure is increased from zero. In other words, the point (hereinafterreferred to as "front-rear force distribution point") representing themaster cylinder pressure and the front wheel braking pressure in thegraph of FIG. 14 is moved from the zero point of the coordinate in thedirection parallel to the horizontal axis along which the mastercylinder pressure is taken.

When the opening pressure difference of the first check valve 72 isexceeded as a result of further depression of the brake pedal 14, theforce distribution point is moved to point "a" indicated in FIG. 14. Ifthe master cylinder pressure is further increased, the first check valve72 is opened, and the brake fluid flows from the master cylinder 10toward the front wheel brake cylinder 20, whereby the front wheelbraking pressure is raised from zero as the master cylinder pressure isincreased, such that the front wheel braking pressure is lower than themaster cylinder pressure by the preset opening pressure difference ofthe first check valve 72. Consequently, the force distribution point ismoved from point "a" to "b" indicated in FIG. 14.

If the brake pedal 14 is released when the master cylinder pressurecorresponds to the distribution point "b", the second check valve 74 isclosed inhibiting the flow of the brake fluid from the front wheel brakecylinder 20 toward the master cylinder 10 as long as the master cylinderpressure is higher than the front wheel braking pressure. Therefore, thefront wheel braking pressure is held constant at the level correspondingto the force distribution point "b", while the master cylinder pressureis lowered to a level corresponding to a force distribution point "c"indicated in FIG. 14.

When the master cylinder pressure is further reduced to a level lowerthan the front wheel braking pressure, the second check valve 74 isopened permitting the brake fluid to flow from the front wheel brakecylinder 20 toward the master cylinder 10, causing the front wheelbraking pressure to be lowered as the master cylinder pressure islowered. Consequently, the force distribution point is moved from "c" tothe zero point.

Referring back to FIG. 12, a solenoid-operated shut-off valve 80 isdisposed in parallel with the check valve device 70. This shut-off valve80 is normally held open, thereby disabling the check valve device 70 toperform its pressure reducing function. The shut-off valve 80 iscontrolled by a controller 84, which principally consists of a computerincorporating a central processing unit (CPU), a read-only memory (ROM)and a random-access memory (RAM). The controller 84 is adapted to closethe shut-off valve 80, for thereby enabling the pressure reducingfunction of the check valve device 70, when the vehicle load exceeds apredetermined value. To this end, a load-sensing switch 90 is connectedto the controller 84. This load-sensing switch 90 is fixed to the sprungmember of the vehicle, namely, to the vehicle body, such that the switch90 is located near the lever 64 of the P valve 34, as indicated in FIG.13. The switch 90 is positioned so that the switch 90 is OFF when thevehicle load is smaller than the predetermined value, and is turned 0Nwhen the vehicle load exceeds the predetermined value, that is, when theangle of counterclockwise pivoting of the lever 64 while pushing thepiston 42 into the valve body 40 exceeds a predetermined value. Thepredetermined value of the vehicle load may be the nominal maximum loadvalue of the vehicle (may correspond to the full-load run), or may be80% or 60% of the nominal maximum load value, for example.

The ROM in the computer of the controller 84 stores a control programfor a pressure reduction control routine as illustrated in the flowchart of FIG. 15, for controlling the solenoid-operated shut-off valve80 so as to disable the check valve device 70 as needed. This routine isexecuted with a predetermined cycle time. The routine is initiated withstep S1 to determine whether the load-sensing switch 90 is in the ONstate. If the switch 90 is not in the ON state due to the vehicle loadsmaller than the predetermined value, a negative decision (NO) isobtained in step S1, and the control flow goes to step S2, in which thesolenoid of the shut-off valve 80 is de-energized to open the shut-offvalve 80, for disabling the check valve device 70 to perform itspressure reducing function. If the vehicle load is larger than thepredetermined value and the switch 90 is in the ON state, an affirmativedecision (YES) is obtained in step S1, and the control flow goes to stepS3 in which the solenoid of the shut-off valve 80 is energized to closethe shut-off valve 80, for enabling the check valve device 70 to performits pressure reducing function. One cycle of execution of the pressurereduction control routine is terminated after the completion of step S2or S3.

Referring to the graph of FIG. 16, one advantageous effect of thepresent braking system will be explained. For clarity, the followingdescription refers only to operation of the braking system during theminimum-load run and the full-load run, as examples. Although notlimited to minimum and full-load runs, no description of the operationof vehicle load's intermediate between the maximum and minimum valueswill be provided. This applies to the other embodiments of the inventiondescribed later.

When the brake pedal 14 is depressed by the vehicle operator while theload-sensing switch 90 is in the OFF state with the vehicle load smallerthan the predetermined value, the master cylinder pressure is applied tothe front wheel brake cylinder 220 without reduction of the mastercylinder pressure by the first check valve 72. At the same time, themaster cylinder pressure is applied to the rear wheel brake cylinder 30through the P valve 34. Consequently, the front-rear force distributionpoint is moved from the zero point along a first basic distribution line(determined by the basic braking arrangement without the P valve 34 andcheck valve device 70) and a distribution line of the P valve 34 for theminimum-load run, as indicated in the graph of FIG. 16. If the vehicleoperator depresses the brake pedal 14 to a position corresponding to thefront wheel braking pressure that is slightly lower than the level atwhich the front wheels begins to be locked on the road surface, theforce distribution point is moved to a point of intersection between thefirst basic distribution line or the distribution line of the P valve 34for the minimum-load run and a front wheel locking line for theminimum-load run (also indicated in FIG. 16). The front wheel lockingline is determined by the friction coefficient of the road surface onwhich the vehicle is running. In the present specific example, the pointof intersection is indicated at "a". As is apparent from the graph ofFIG. 16, the rear wheel braking force corresponding to the intersectionpoint "a" is almost as large as the level represented by the idealdistribution curve for the minimum-load run, and the vehicle can bebraked with the front and rear braking forces being suitably controlled.

When the vehicle is in the full-load run with the load-sensing switch 90placed in the ON position, on the other hand, the master cylinderpressure is reduced by the first check valve 72 so that the reducedmaster cylinder pressure is applied to the front wheel brake cylinder20, while the master cylinder pressure is applied to the rear wheelbrake cylinder 30 through the P valve 34. In this case, the front-rearforce distribution point is first moved from the zero point to point "b"in the direction parallel to the vertical axis along which the rearwheel braking force is taken in the graph FIG. 16. That is, only therear wheel braking pressure or force is increased while the front wheelbraking pressure or force is kept at zero, until the master cylinderpressure is raised to create the preset opening pressure difference ofthe first check valve 72.

When the master cylinder pressure has been raised to create the presetopening pressure difference of the first check valve 72, the front wheelbraking pressure begins rise and the force distribution point is movedfrom the point "b" along a second basic distribution line (determined bythe basic braking arrangement without the P valve 34 but with the checkvalve device 70) and a distribution line of the P valve 34 for thefull-load run, as also indicated in FIG. 16. If the vehicle operatordepresses the brake pedal 14 to a position corresponding to the frontwheel braking pressure that is slightly lower than the level at whichthe front wheels begins to be locked on the road surface, the forcedistribution point is moved to a point of intersection between thesecond basic distribution line or the distribution line of the P valve34 for the full-load run and a front wheel locking line for thefull-load run (also indicated in FIG. 16). In the present specificexample, the point of intersection is indicated at "c".

As described above, the present braking system is adapted such that thedistribution of the front and rear wheel braking forces during theminimum-load run is controlled according to a combination of the firstbasic distribution line and the distribution line of the P valve 34 forthe minimum-load run, while the distribution during the full-load run iscontrolled according to a combination of the second basic distributionline and the distribution line of the P valve 34 for the full-load run.Accordingly, the present braking system is effective to reduce theamount of deviation of the actual rear wheel braking force with respectto the ideal distribution curve during the full-load run, as comparedwith the conventional braking system in which the distribution line ofthe P valve for the full-load run is located below the correspondingdistribution line of the P valve 34 according to the present invention.Consequently, the braking system according to the present embodiment iscapable of producing an increased sum of the front and rear wheelbraking forces (i.e., increased total braking force), which makes itpossible to reduce the required braking distance of the vehicle.

In the present embodiment, the distribution line of the P valve 34 forthe minimum-load run corresponds to a first distribution pattern of thefront and rear wheel braking forces during the minimum-load run of thevehicle, while the distribution line of the P valve 34-for the full-loadrun according to the invention corresponds to a second distributionpattern of the front and rear wheel braking forces during the full-loadrun of the vehicle. It will be understood from the graph of FIG. 16 thatthe second distribution pattern defines the rear wheel braking forcelarger than that defined by the first distribution pattern, over theentire ranges of the front and rear wheel braking forces.

It is also noted that the rear wheel braking force defined by the seconddistribution pattern is larger than that defined by the firstdistribution pattern, by the same amount corresponding to the openingpressure difference of the first check valve 72, over the entire rangebetween zero and the value corresponding to the threshold level of the Pvalve 34 during the minimum-load run. That is, the rear wheel brakingforce according to the second distribution pattern is raised toward thelevel of the ideal distribution curve (for the full-load run), withrespect to that according to the first distribution pattern line, evenwhen the rear wheel braking force is near zero. Theoretically,therefore, there is a front-rear distribution area in which the rearwheels may be locked on the road surface before the front wheels.However, the friction coefficient of the road surface on which thevehicle actually runs does not vary over the entire range between 0and 1. In practice, the road surface may not have a friction coefficientextremely close to 0, for example, within a range of 1-0.05 whichcorresponds to the theoretical distribution area in which the priorlocking of the rear wheels may occur. Therefore, the prior locking ofthe rear wheels will not actually occur if the opening pressuredifference of the first check valve 72 (which determines the differenceof the rear wheel braking force according to the second distributionpattern from that according to the first distribution pattern) isdetermined so that the rear wheel braking force according to the seconddistribution pattern does not exceed that according to the idealdistribution curve for the full-load run.

In the present embodiment, the opening pressure difference of theopening pressure of the first check valve 72 is preset at a value notlarger than a value corresponding to the amount of increase of the rearwheel braking force according to the second distribution pattern withrespect to that according to the first distribution pattern, when thesecond distribution pattern passes a point of simultaneous locking ofthe front and rear wheels on the road surface which has the practicallylowest friction of coefficient value.

Upon releasing of the brake pedal 14, the brake fluid in the front wheelbrake cylinder 20 is returned to the master cylinder 10 through thesecond check valve 74, while the brake fluid in the rear wheel brakecylinder 30 is returned to the master cylinder through the P valve 34.Thus, the brake fluid can be discharged from the front wheel brakecylinder 20 toward the master cylinder 10, irrespective of the currentlyselected position of the shut-off valve 80, when the brake pedal 14 isreleased.

It will be understood from the foregoing description of the presentembodiment that the lever 64, load-sensing switch 90, shut-off valve 80and controller 84 cooperate with each other to constitute one example ofselective disabling means for selectively disabling the check valvedevice 70, more particularly, the first check valve 72. This selectivedisabling means cooperates with the check valve device 70 and the Pvalve 34 to constitute an example of a distribution control device forcontrolling the distribution of the braking forces to be applied to thefront and rear wheels, according to a selected one of the first andsecond distribution patterns indicated above.

Referring next to FIGS. 17-22, there will be described a secondembodiment of the present invention in the form of an anti-lock brakingsystem of the X-cross type, which has various features.

Like the braking system according to the first embodiment, the brakingsystem according to the present second embodiment is of the diagonal orX-crossing type. However, the present braking system is different fromthe braking system of the first embodiment, in that the present brakingsystem is capable of effecting an anti-lock control of the brakingpressures or forces of the front and rear wheels, and is adapted to usenot only the master cylinder but also the fluid recirculating pump asthe hydraulic pressure source. These aspects of the present embodimentwill be described in detail.

As shown in FIG. 17, a normally-open first solenoid-operated shut-offvalve 100 is disposed in a portion of the front brake cylinder passage22 between the master cylinder 10 and the point of connection of thefront and rear brake cylinder passages 22, 24. Further, a by-pass returnpassage 102 is provided in parallel with the first shut-off valve 100,so as to by-pass the shut-off valve 100. The by-pass return passage 102is provided with a check valve 104, which inhibits a flow of the brakefluid in a direction from the master cylinder 10 toward the front wheelbrake cylinder 20, and permits a flow of the brake fluid in the reversedirection with the valve opening pressure difference being substantiallyzero.

A proportioning valve or P valve 110 is provided in the rear brakecylinder passage 24. Unlike the P valve 34 used in the first embodiment,the P valve 110 is not of a load-sensing type having a variablethreshold pressure for initiation of the reducing function, but of afixed threshold type wherein the pressure reducing function is initiatedat a predetermined threshold level of the master cylinder pressure. TheP valve 110 will be described in detail

The P valve 110 has a housing 112, which has a stepped cylinder bore 118with a large-diameter portion 114 and a small-diameter portion 116. Astepped valve piston 124 having a large-diameter portion 120 and asmall-diameter portion 122 is slidable received in the stepped cylinderbore 118. The valve piston 124 is biased by biasing means in the form ofa spring 126 so that the piston 124 is normally held in a non-operatedposition in which the end face of the large-diameter portion 120 abutson the bottom wall of the small-diameter portion 116 of the housing 112.Between the cylinder bore 118 and the valve piston 124, there isdisposed a sealing member in the form of a cup seal 128. This cup seal128 divides the space within the cylinder bore 118 into two sections.One of these two sections, which is on the side of the large-diameterportion 114, serves as an input chamber 130, while the other section onthe side of the small-diameter portion 116 serves as an output chamber132. The input chamber 130 is connected to the master cylinder 10, whilethe output chamber 132 is connected to the rear wheel brake cylinder 30.

The cup seal 128 consists of a one-way sealing portion 134 and a two-waysealing portion 136. The one-way sealing portion 134 inhibits a flow ofthe brake fluid in the direction from the input chamber 130 toward theoutput chamber 132. The one-way sealing portion 134 is in fluid-tightcontact with the circumferential surface of the large-diameter portion114 of the cylinder bore 118. The one-way sealing portion 134 permits aflow of the fluid in the direction from the output chamber 132 towardthe input chamber 130 while the sealing portion 134 is spaced apart fromthe surface of the large-diameter portion 114. When the valve piston 124is moved from the non-operated position of FIG. 18 to an operatedposition (in the right direction as seen in the figure), the shouldersurface between the large-diameter and small-diameter portions 120, 122of the piston 124 is brought into abutting contact with the two-waysealing portion 136, thereby inhibiting flows of the fluid in theopposite directions between the input and output chambers 130, 132. Whenthe valve piston 124 is placed in the non-operated position of FIG. 18,the two-way sealing portion 136 is unseated off the shoulder surface ofthe piston 124. This permits the fluid to flow between the input andoutput chambers 130, 132.

The cup seal 128 has an annular protrusion formed on each of theopposite surfaces which define the input and output chambers 130, 132.The annular protrusions have a semi-circular cross sectional shape asseen in FIG. 18. The annular protrusion on the side of the input chamber130 prevents the cup seal 128 from contacting the valve piston 124 atthe entire area of the surface on the side of the input chamber 130,while the annular protrusion on the side of the output chamber 132prevents the cup seal 128 from contacting the shoulder surface betweenthe large-diameter and small-diameter portions 120, 122 of the cylinderbore 118, at the entire area of the surface on the side of the outputchamber 132.

As shown in FIG. 17, a normally-open second solenoid-operated shut-offvalve 140 is disposed in a portion of the rear brake cylinder passage 24between the P valve 110 and the point of connection of the front andrear brake cylinder passages 22, 24. A reservoir passage 142 isconnected at one end thereof to a portion of the rear brake cylinderpassage 24 between the P valve 110 and the second shut-off valve 140,and at the other end to a reservoir 144. A normally-closed thirdsolenoid-operated shut-off valve 146 is provided in the reservoirpassage 142. In the present second embodiment, the first shut-off valve100 is an example of a master cylinder cut-off valve, the secondshut-off valve 140 is an example of an intermediate valve, and the thirdshut-off valve 146 is an example of a pressure reducing valve.

A pump passage 148 is connected at one end thereof to the reservoir 144and at the other end to the rear brake cylinder passage 24. A pump 150,which is provided in the pump passage 148, creates pressure and vacuumfor the brake fluid in the reservoir 144. The pump 150 is a plunger-typedriven by a motor 152 to deliver the pressurized fluid in anintermittent manner. The output, or delivery, end (the other endindicated above) of the reservoir passage 148 is connected to a portionof the rear brake cylinder passage 24 on the upstream side of the secondshut-off valve 140, namely, on the side of the master cylinder 10.

A return passage 154 is connected at one end thereof to a portion of therear brake cylinder passage 24 between the P valve 110 and the secondshut-off valve 140, and at the other end to a portion of the front brakecylinder passage 22 between the master cylinder 10 and the firstshut-off valve 100. A check valve 156 is provided in the return passage154. This check valve 156 inhibits flow of the brake fluid in adirection from the master cylinder 10 toward the rear wheel brakecylinder 30 and permits flow of the fluid in the reverse direction withthe valve opening pressure difference being substantially zero.

A check valve device 160 is disposed in a portion of the rear brakecylinder passage 24 between the point of connection of the rear brakecylinder passage 24 and the pump passage 148 and the point of connectionof the front and rear brake cylinder passages 22, 24. Like the checkvalve device 70 provided in the first embodiment, the check valve device160 includes a first check valve 162 whose opening pressure differenceis not substantially zero, and a second check valve 164 whose openingpressure difference is substantially zero. The first and second checkvalves 162, 164 are disposed in parallel with each other and haveopposite directions in which the brake fluid is permitted to flow.

However, the check valve device 160 is different from the check valvedevice 70 in the location and the orientation of the first and secondcheck valves. In the first embodiment, the master cylinder 10 alwaysserves as the hydraulic pressure source for the front and rear wheelbrake cylinders 20, 30, and therefore the check valve device 70 isinterposed between the master cylinder 10 and the front wheel brakecylinder 20. In the present second embodiment, on the other hand, thepump 150 serves as the hydraulic pressure source when the wheel brakingpressures are controlled in the anti-lock manner, and therefore thefirst check valve 162 is oriented to allow the first or pressurereducing check valve 162 to serve as a check valve which permits a flowof the brake fluid in the direction from the pump 150 toward the frontwheel brake cylinder 20 after the pressure of the brake fluid deliveredfrom the pump 150 becomes higher than the pressure in the front wheelbrake cylinder 20 by more than a predetermined or preset openingpressure difference of the check valve 162. The principle of operationof the pump 150 as the hydraulic pressure source will be described belowin detail.

FIG. 19 illustrates the flow of the brake fluid to and from the mastercylinder 10, pump 150 and front and rear wheel brake cylinders 20, 30.It should be noted that FIG. 19 schematically shows a major portion ofthe brake fluid circuit, but ignores the provisions for both the P valve110 and the normally-open second shut-off valve 140.

During a normal operation of the present braking system without anoperation of the pump 150, the brake fluid pressurized by the mastercylinder 10 is supplied to the front wheel brake cylinder 20 through thefirst shut-off valve 100, and to the rear wheel brake cylinder 30through both the first shut-off valve 100 and the second check valve 164of the check valve device 160. Since the opening pressure difference ofthe second check valve 166 is substantially zero, almost the samebraking pressures are applied to the front and rear wheel brakecylinders 20, 30.

When the pump 150 is operated, on the other hand, the first shut-offvalve 100 is closed, and the brake fluid delivered from the pump 150 issupplied to the front wheel brake cylinder 220 through the first checkvalve 162, and supplied to the rear wheel brake cylinder 30 withoutflowing through the first check valve 162. Since the opening pressuredifference of the first check valve 162 is not substantially zero, thebraking pressure in the front wheel brake cylinder 20 is made lower thanthat in the rear wheel brake cylinder 30 by an amount corresponding tothe preset opening pressure difference of the first check valve 162.

The constructions of the first and second check valves 162, 164 will bedescribed below in detail.

The solenoids of the first, second and third solenoid-operated shut-offvalves 100, 140 and 146 are connected to a controller 170, whichprincipally consist of a computer, A/D converters, and drivers. Thecomputer includes a central processing unit (CPU), read-only memory(ROM), random-access memory (RAM) and a bus. The controller 170selectively opens and closes the shut-off valves 100, 140, 146 asneeded, depending upon the output signals of wheel speed sensors 172,174, which represent the rotating speeds of the front and rear wheels.

The motor 152 for driving the pump 150 is also controlled by thecontroller 170. In principle, the motor 152 and the pump 150 are turnedoff when the entire volume of the brake fluid in the reservoir 144 hasbeen pumped up by the pump 150. While the motor 152 may be operated aslong as the wheel braking pressures are controlled in the anti-lockmanner, the pump 150 is stopped upon evacuation of the reservoir 144 tominimize operating noise.

The evacuation of the reservoir 144 may be detected directly by aposition sensor (e.g., proximity switch) adapted to detect the axialposition of a piston 176 of the reservoir 144, or indirectly by a loadsensor adapted to detect a load acting on the motor 152 on the basis ofan electric current applied to the motor 152, or by a timer adapted tomeasure the time of continuous operation of the motor 152. Whereevacuation is indirectly detected, the motor 152 is turned off when thedetected load is lowered below a predetermined threshold value or whenthe measured operation time exceeds a predetermined limit.

The controller 170 may be adapted to turn off the motor 152 and stop thepump 150 when it is required to discharge the brake fluid from both ofthe front and rear wheel brake cylinders 20, 30 for rapidly lowering thebraking pressures during an anti-lock control of the braking pressures.In this respect, it is noted that an operation of the pump 150 todeliver the pressurized fluid to the rear brake cylinder passage 24prevents reduction in the braking pressures in both of the front andrear wheel brake cylinders 20, 30.

Further, the controller 170 may be adapted to stop the pump 150 when itis required to increase the braking pressures in the front and rearwheel brake cylinders 20, 30 by only a small amount. Since the pump 150of the plunger type delivers the brake fluid intermittently at a certaincycle time, it is difficult to increase the braking pressures by anamount which is smaller than the amount of increase of the brakingpressures by one delivery of the fluid from the pump 150. Therefore,when only a small amount of increase of the braking pressures isrequired, the pump 150 is stopped, and the first shut-off valve 100 isopened for a short length of time to permit the master cylinder pressureto be applied to the front and rear wheel brake cylinders 20, 30,thereby raising the braking pressures by the desired small amount.

There will next be described in detail operation of the controller 170to control the shut-off valves 100, 140 and 146.

During braking of the vehicle, the controller 100 monitors the rotatingconditions (e.g., deceleration values, slip amounts, and slip ratios) ofthe vehicle's individual wheels by receiving the output signals of thewheel speed sensors 172, 174, and determines whether any wheels have alocking tendency. The controller 170 controls the shut-off valves 100,140, 146 in a selected one of seven pressure control modes as indicatedin TABLE 1 below, to control the wheel brake cylinders 20, 30. Theseseven pressure control modes are established by respective differentcombinations of the open and closed states of the three shut-off valves100, 140, 146.

                  TABLE 1                                                         ______________________________________                                        States of       Pressure Control States of                                    Shut Off        Front and Rear Brake                                          Valves          Cylinders                                                     Mode  1       2     3     Front      Rear                                     ______________________________________                                        1     O       O     C     M/C Increase                                                                             M/C Increase                             2     O       C     C     M/C Increase                                                                             Hold                                     3     O       C     O     M/C Increase                                                                             Reduction                                4     C       O     C     Pump Increase                                                                            Pump Increase                            5     C       C     C     Pump Increase                                                                            Hold                                     6     C       C     0     Pump Increase                                                                            Reduction                                7     C       O     O     Reduction  Reduction                                ______________________________________                                    

To effect the anti-lock control of the wheel braking pressures, thecontroller 170 performs the following steps: (a) determining whether anyone of the front and rear wheels of the two pressure applicationsub-systems has a locking tendency, and if it is determined that anywheel has a locking tendency, determining on the basis of the rotatingcondition of that wheel a pressure control command (selected from amonga pressure reducing command, a pressure holding command and a pressureincreasing command) which should be generated to control the brakingpressure in the brake cylinder of the wheel in question; (b) thenselecting one of the seven pressure control modes depending upon thedetermined pressure control command (pressure reducing, holding orincreasing command) and depending upon whether the wheel having thelocking tendency is a front or rear wheel; and (c) then controlling thepressure in the brake cylinder of the wheel in question in the selectedpressure control mode. To this end, the ROM of the controller 170 storesroutines for determining the pressure control commands for theindividual wheels on the basis of the rotating conditions of the wheels,and routines for controlling (turning on or off) the solenoids of therespective shut-off valves 100, 140, 146 according to the determinedpressure control commands.

The anti-lock pressure control operation of the present braking systemwill be described in detail, assuming that the front wheel associatedwith one of the two pressure application sub-systems has a lockingtendency without a locking tendency of the rear wheel.

In this case, the pressure in the front wheel brake cylinder 20 shouldfirst be reduced. However, as is apparent from TABLE 1, the sevenpressure control modes available do not include a mode for reducing onlythe pressure in the front wheel brake cylinder 20. Therefore, theseventh pressure control mode is selected to reduce the pressures inboth of the front and rear wheel brake cylinders 20, 30.

In the seventh pressure control mode, the solenoid of the first shut-offvalve 100 is turned ON to close shut-off valve 100. This disconnects thefront and rear wheel brake cylinders 20, 30 from the master cylinder 10.Further, the solenoid of the third shut-off valve 146 is turned ON toopen this valve 146. This reduces the pressures in the front and rearwheel brake cylinders 20, 30. Described more specifically, the frontwheel brake cylinder 20 is brought into communication with the reservoir144 through the second check valve 164, normally-open second shut-offvalve 140, and now opened third shut-off valve 146, whereby the brakefluid is permitted to flow from the front wheel brake cylinder 20 to thereservoir 144. At the same time, the rear wheel brake cylinder 30 iscommunicated with the reservoir 144 through the P valve 110 and theopened third shut-off valve 146, and the brake fluid is permitted toflow from the rear wheel brake cylinder 30 to the reservoir 144. Thus,the braking pressures in both of the front and rear wheel brakecylinders 20, 30 are reduced in the seventh pressure control mode.

The seventh pressure control mode of operation is terminated when thelocking tendency of the front wheel is eliminated or considerablyreduced as a result of the reduction in the wheel brake cylinders 20,30. Then, the pressure in the front and rear wheel brake cylinders 20,30 are controlled in a selected one of the fourth, fifth, sixth andseventh pressure control modes, depending upon the locking tendencies ofthe front and rear wheels.

In the fourth pressure control mode, the first and third shut-off valves100, 146 are both closed, while the second shut-off valve 140 is opened,so that the fluid delivered from the pump 150 is returned to the frontwheel brake cylinder 20 through the first check valve 162, and to therear wheel brake cylinder 30 through the opened second shut-off valve140 and the P valve 110, whereby the pressures in the front and rearwheel brake cylinders 20, 30 are both increased. In this fourth pressurecontrol mode, the pressure of the fluid delivered from the pump 150 isreduced by the first check valve 162 by the preset opening pressuredifference of the check valve 162. Therefore, the braking pressure inthe front wheel brake cylinder 20 is lower than the braking pressure inthe rear wheel brake cylinder 30, by the opening pressure difference ofthe first check valve 162, when the braking pressures are increased.

In the fifth pressure control mode, the three shut-off valves 100, 140,146 are all closed, and the pressure in the front wheel brake cylinder20 is increased by operation of the pump 150 as in the fourth mode,while the pressure in the rear wheel brake cylinder 30 is held constant.

In the fifth pressure control mode, the brake fluid delivered from thepump 150 is not returned to the rear wheel brake cylinder 30, but isreturned only to the front wheel brake cylinder 20. In the fourth mode,on the other hand, the brake fluid from the pump 150 is also returned tothe rear wheel brake cylinder 30. Accordingly, the rate of increase inthe pressure in the front wheel brake cylinder 20 is higher in the fifthmode than in the fourth mode, as indicated in FIG. 20. As also shown inthis figure, the pressure in the rear wheel brake cylinder 30 isincreased in the fourth mode while the pressure in the same cylinder isheld constant in the fifth mode.

In the sixth pressure control mode, the first and second shut-off valves100, 140 are both closed while the third shut-off valve 146 is opened,whereby the pressure in the front wheel brake cylinder 20 is increasedas in the fourth mode, while the pressure in the rear wheel brakecylinder 30 is reduced.

According to these principles the first, second, and third pressurecontrol modes are not used for the anti-lock control of the brakingpressure of the front wheel brake cylinder 20. In these three modes, thefirst shut-off valve 100 is opened. During the anti-lock pressurecontrol, it is desirable to disconnect the front and rear wheel brakecylinders 20, 30 from the master cylinder, in order to reduce thedelivery pressure of the pump 150 and minimize the pressure pulsation ofthe fluid delivered from the pump 150. However, if it becomes necessaryto increase the rear wheel brake cylinder 20, 30 after the reservoir 144is entirely evacuated with the entire volume of the fluid pumped up bythe pump 150, an appropriate one of the first, second and third pressurecontrol modes is established to increase the pressure in the wheel brakecylinder in question with the pressure generated by the master cylinder10.

When the pressure in the front wheel brake cylinder 20 is increased byoperation of the pump 150 in the fourth or fifth pressure control mode,the check valve 104 functions as a pressure relief valve to prevent thefront wheel braking pressure from exceeding the master cylinderpressure.

The braking system operates with front wheel locking tendency withoutrear wheel locking tendency. As will be described below, the brakingsystem also operates with a locking tendency of the rear wheel without alocking tendency of the front wheel.

In this case, it is necessary to first reduce the pressure in the rearwheel brake cylinder 30. To this end, the braking system is first placedin the third pressure control mode wherein the first and third shut-offvalve 100, 146 are opened while the second shut-off valve 140 is closed,whereby substantially no anti-lock pressure control is effected withrespect to the pressure in the front wheel brake cylinder 20. That is,the pressure in the front wheel brake cylinder 20 is increased by thepressure generated by the master cylinder 10, while the pressure in therear wheel brake cylinder 30 is reduced through the opened thirdshut-off valve 146.

Subsequently, the first through seventh pressure control modes areselectively established by the controller 170 as needed. While the frontwheel does not have a locking tendency, the first, second and thirdpressure control modes are selectively established, and only thepressure in the rear wheel brake cylinder is controlled in the anti-lockmanner. If the front wheel as well as the rear wheel has a lockingtendency, or if only the front wheel has a locking tendency with thelocking tendency of the rear wheel being eliminated, the front and rearwheel braking pressures or the front wheel braking pressure is/arecontrolled in the anti-lock manner as in the case where the front wheelhas a locking tendency without a locking tendency of the rear wheel.

The rear wheel has a locking tendency without a locking tendency of thefront wheel if the front wheel lies on an area of an unevenfriction-coefficient road surface which area has a relatively highfriction coefficient, while the rear wheel lies on an area of the roadsurface having a relatively low friction coefficient. In this case, itis preferable to maximize the front wheel braking pressure whilepreventing the locking of the front wheel, so that the relatively highfriction coefficient of the road surface area is utilized by the frontwheel to reduce the braking distance of the vehicle. On the other hand,it is preferable to maximize the cornering force acting on the rearwheel, for improving the steering or directional stability of thevehicle. In other words, it is desired that the braking system becapable of increasing the front wheel braking pressure withoutincreasing the rear wheel braking pressure, or capable of reducing therear wheel braking pressure without increasing the front wheel brakingpressure. In the present second embodiment, the fifth or sixth pressurecontrol mode is established to increase the front wheel braking pressurewithout an increase in the rear wheel braking pressure, and the sixthpressure control mode is established to reduce the rear wheel brakingpressure without a decrease in the front wheel braking pressure. Thus,the present embodiment assures not only reduction in the requiredbraking distance of the vehicle but also an improvement of the steeringstability of the vehicle in the case of braking of the vehicle while thefront wheel is on the high friction-coefficient area of an unevenfriction-coefficient road surface while the rear wheel is on the lowfriction-coefficient area.

An advantageous effect of the anti-lock braking system according to thesecond embodiment will be described by reference to the graph of FIG.21.

In a normal braking of the vehicle initiated by depression of the brakepedal 14 by the vehicle driver, the master cylinder 10 rather than thepump 150 functions as the pressure source, and the master cylinderpressure is applied to the front wheel brake cylinder 20, irrespectiveof the existence of the check valve device 160. Accordingly, thefront-rear force distribution point is moved from the zero point of thecoordinate system of the graph of FIG. 21, along a first basicdistribution line and along a distribution line of the P valve 110 forthe minimum-load run of the vehicle.

If the vehicle is in the minimum-load run (one form of a vehicle runwith a relatively small load), the force distribution point reaches apoint "a" as indicated in FIG. 21, when the depression force acting onthe brake pedal 14 has been increased to increase the front wheelbraking pressure to a level slightly lower than a level at which thefront wheel begins to be locked on the road surface. When the anti-lockcontrol of the front wheel braking pressure is started due to anexcessive degree of locking of the front wheel as a result of a furtherincrease in the depression force of the brake pedal 14, both the frontwheel braking pressure and the rear wheel braking pressure are reducedin the seventh pressure control mode. Consequently, the forcedistribution point is moved from the point "a" in the left direction, asindicated in the graph of FIG. 21, to a point which lies on the firstbasic distribution line or the distribution line of the P valve 110 forthe minimum-load run. In this specific example, the force distributionpoint is moved to point "b". The above explanation is based on anassumption that the brake pedal 14 is held depressed and the mastercylinder pressure is continuously increased even after the firstreduction of the front and rear wheel braking pressures is started,namely, an assumption that the delivery pressure of the pump 150 isincreased from the level at the time of start of the first reduction ofthe wheel braking pressures.

If the fourth pressure control mode of operation is initiated toincrease the front and rear wheel braking pressures as a result ofelimination of the locking tendency of the front wheel, the brake fluiddelivered from the pump 150 is supplied to the front wheel brakecylinder 20, with the pressure reduction corresponding to the openingpressure difference of the first check valve 162. However, the brakefluid delivered from the pump 150 is supplied to the rear wheel brakecylinder 30, without the pressure reduction. It is noted that somevolume of brake fluid has been stored in the reservoir 144 by the timethe locking tendency of the front wheel has been eliminated when (i.e.,the fourth pressure control mode of operation is initiated). After thedelivery of the brake fluid from the pump 150 is started, the frontwheel braking pressure and force are held constant and only the rearwheel braking pressure and force are increased, until the first checkvalve 162 is opened. Accordingly, the force distribution point is movedfrom the point "b" in the positive direction along the vertical axis ofthe graph of FIG. 21 (along which the rear wheel braking force istaken), to a point which lies on a second basic distribution line or adistribution line of the P valve 110 for the full-load run. In thisspecific example, the pressure distribution point is moved to point "c".Subsequently, the force distribution point is moved from point "c",along the distribution line of the P valve 110 for the full-load run, inthe direction of increasing the rear wheel braking force, and eventuallyreaches point "d" of intersection between a rear wheel locking line forthe minimum-load run and the distribution line of the P valve 110 forthe full-load run. Then, the rear wheel braking pressure is controlledin the anti-lock manner so as to eliminate the locking tendency of therear wheel.

If the vehicle is in the full-load run (one form of a vehicle run with arelatively large load), the front-rear force distribution point is movedto point "e" as indicated in FIG. 21, when the depression force actingon the brake pedal 14 is increased to increase the front wheel brakingforce to a level slightly lower than the wheel locking level. A furtherincrease in the brake pedal depression force will cause initiation ofcontrol of the front wheel braking pressure in the anti-lock manner. Asa result, the force distribution point is moved to point "b" asindicated above.

When the fourth pressure control mode of operation is initiated toincrease the front and rear wheel braking pressures as a result ofelimination of the locking tendency of the front wheel, the forcedistribution point is moved from point "b" in the positive direction ofthe vertical axis to point "c" as in the above case. With a furtherincrease in the front and rear wheel braking pressures by operation ofthe pump 150, the force distribution point is further moved from point"c" along the distribution line of the P valve 110 for the full-loadrun, and eventually reaches point "f" of intersection between thedistribution line of the P valve 110 for the full-load run and a frontwheel locking line for the full-load run. Thereafter, the anti-lockcontrol is effected to eliminate the locking tendency of the frontwheel.

In the present second embodiment, the actual distribution of the frontand rear wheel braking forces is controlled, during the normal pressurecontrol operation (without the anti-lock control of the brakingpressures), according to a combination of the first basic distributionline and the distribution line of the P valve 110 for the minimum-loadrun, irrespective of whether the vehicle is in the minimum-load run orfull-load run. During the anti-lock pressure control operation with thevehicle in the minimum-load run, on the other hand, the actualdistribution of the front and rear wheel braking forces is generallycontrolled according to the ideal distribution curve for theminimum-load run, and more precisely, according to a portion of theideal distribution curve for the minimum-load run which is located below(in FIG. 21) the distribution line of the P valve 110 for the full-loadrun, and a portion of the distribution line of the P valve 110 for thefull-load run which is located below the ideal distribution curve forthe minimum-load run. During the anti-lock pressure control operationwith the vehicle in the full-load run, the actual distribution isgenerally controlled according to the distribution line of the P valvefor the full-load run, and more precisely, according to a portion of theideal distribution curve for the full-load run which is located belowthe distribution line of the P valve for the full-load run, and aportion of the distribution line of the P valve for the full-load runwhich is located below the ideal distribution curve for the full-loadrun.

In the present second embodiment, therefore, the sum of the front andrear wheel braking forces or the total wheel braking force during thefull-load vehicle run is increased with respect to that in theconventional braking system in which the anti-lock pressure controloperation is effected according to the first basic distribution lineeven during the full-load vehicle run. Accordingly, the required brakingdistance of the vehicle can be reduced in the present braking system. Itwill also be understood from the graph of FIG. 21 that the rear wheelbraking pressure or force during the full-load vehicle run can beeffectively increased to thereby reduce the required vehicle brakingdistance, even when the ideal rear braking force is smaller than thethreshold level of the P valve 110, for example, when the fully-loadedvehicle is abruptly braked on a snow-covered road surface or other roadsurface having a low friction coefficient.

In the present embodiment, the distribution of the front and rear wheelbraking forces during the normal pressure control operation and duringthe anti-lock pressure control operation and the minimum-load vehiclerun corresponds to the first distribution pattern, while thedistribution of the front and rear wheel braking forces during theanti-lock pressure control operation and the full-load vehicle runcorresponds to the second distribution pattern. The second distributionpattern defines the rear wheel braking force larger than that defined bythe first distribution pattern, over the entire ranges of the front andrear wheel braking forces.

In the present second embodiment, the opening pressure difference of thefirst check valve 162 is larger than that of the first check valve 72provided in the first embodiment of FIG. 12. Described moreparticularly, the opening pressure difference of the first check valve162 is determined or preset so that the second basic distribution linecan intersect the rear wheel locking line which is determined for thefull-load vehicle run on a road surface having a low frictioncoefficient (about 0.3), such as a road surface covered by compressedsnow. In other words, the opening pressure difference of the first checkvalve 162 is a relatively high value, so that the rear wheel would belocked on such low friction-coefficient road surface upon excessivedepression of the brake pedal 14 during the full-load vehicle run whilethe rear wheel braking pressure is lower than the threshold level of theP valve 110, if the rear wheel braking pressure was not controlled inthe anti-lock manner. In practice, however, early locking of the rearwheel upon excessive depression of the brake pedal under such conditionsis prevented or restricted by the anti-lock pressure control operation.

Namely, the present embodiment adapts the anti-lock pressure controloperation to effectively combine with rear wheel braking pressureshigher than front wheel braking pressures that result from the openingpressure difference of the first check valve 162. This results in anactual front-rear force distribution pattern that is sufficiently closeto the ideal distribution curve for the full-load run, even while therear wheel braking pressure is lower than the threshold level of the Pvalve 110. In addition, the required vehicle braking distance can besignificantly reduced during the full-load vehicle run.

Thus, the anti-lock braking system according to the second embodiment iscapable of reducing the required vehicle braking distance during thefull-load run, not only during braking with a relatively large brakingforce on a relatively high friction-coefficient road surface, but alsoduring braking with a relatively small braking force on a relatively lowfriction-coefficient road surface. For effectively reducing the requiredbraking distance during the full-load vehicle run with a relativelylarge braking force, it is essential that the master cylinder pressurebe increased even after the initiation of the anti-lock pressurecontrol, so that the maximum amount of increase in the front and rearwheel braking pressures is sufficiently large. For this reason, therequired braking distance during the full-load vehicle run with arelatively large braking force cannot always be effectively reduced. Toeffectively reduce the required braking distance during the full-loadvehicle run with a relatively small braking force, on the other hand,the master cylinder pressure need not be increased even after theinitiation of the anti-lock pressure control. Therefore, the requiredbraking distance during the full-load vehicle run with a relativelysmall braking force can always be effectively reduced.

Referring next to FIG. 22, the construction of the check valve device160 will be discussed. In the present embodiment, the check valve device160 and the second shut-off valve 140 are constructed as a unit, asindicated in FIG. 17 by a square block delineated as a one-dot dashedline. This reduces the required number of components and the overallsize of the unit.

The second shut-off valve 140 is provided in a housing 177. As wellknown in the art, the second shut-off valve 140 includes a solenoid 140aand a drive member 140b that consists of a rod extending through thesolenoid 140a in concentric relation with the solenoid 140a. The drivemember 140b is supported by a stationary support member 140c in the formof a sleeve, wherein the drive member 140b is slidably movable in thelongitudinal direction relative of the support member 140c. The drivemember 140b has a partially spherical upper end portion 140d (as seen inFIG. 22) which serves as a valve member, while a valve seat member 140ethat consists of a sleeve is fixed concentrically to the support member140c. The valve seat member 140e has an annular valve seat 140f at theend opposite to the valve member (upper end portion) 140d. The valveseat 140f cooperates with the valve member 140d to constitute a shut-offvalve. The valve seat member 14Oe has a central communication passage140g formed therethrough in the longitudinal direction. Thecommunication passage 140g is open at the valve seat 140f.

The housing 177 indicated above also has a passage 178 having a circularcross section for connecting the front wheel brake cylinder 20 and therear wheel brake cylinder 30 (P valve 110). The valve seat member 140eis fixedly disposed in the passage 178 such that the valve seat member140e is concentric with the passage 178 and such that there is left anannular gap between the valve seat member 140e and the circumferentialsurface of the passage 178. This annular gap functions as an annularpassage 179 concentric with the central communication passage 140gformed through the valve seat member 140e.

Within the annular passage 179, there is disposed a one-way sealingmember in the form of a cup seal which serves as the second check valve164, and a two-way sealing member in the form of an O-ring 140h. The cupseal (second check valve) and the O-ring 140h are arranged in the orderof description in the direction from the front wheel brake cylinder 20toward the rear wheel brake cylinder 30. The cup seal and O-ring 140idivide the annular passage 179 into a first portion on the side of thefront wheel brake cylinder 20, a second portion on the side of the rearwheel brake cylinder 30, and a third intermediate portion on 140ibetween the first and second portions. The intermediate portion 140i isconnected to the output or delivery end of the pump passage 148. Thevalve seat member 140e has a passage 140j for fluid communicationbetween the communication passage 140g and the annular passage 179. In aportion of the communication passage 140g between the point ofconnection to the passage 140j and the end of the passage 140g on theside of the front wheel brake cylinder 20, there exists the first checkvalve 162 that comprises of a check valve with a spring-biased ball140k. The ball 140k is normally held seated on a circular valve seat140m (also formed on the valve seat member 140e) under a biasing actionof biasing means in the form of a spring 140l.

In the check valve device 160 constructed as described above, the firstcheck valve 162 inhibits a flow of the brake fluid in the direction fromthe front wheel brake cylinder 20 toward the intermediate portion 140iof the annular passage 179. However, the second check valve 164 permitsa flow of the brake fluid into the intermediate portion 140i with theopening pressure difference being substantially zero. Further, the firstcheck valve 162 permits brake fluid flow in the direction from theintermediate portion 140i toward the front wheel brake cylinder 20, whenthe pressure in the intermediate portion 140i is higher than thepressure in the front wheel brake cylinder 20 by more than a presetopening pressure difference of the first check valve 162. This openingpressure difference is determined by a biasing force of the spring 1401.The second check valve 164 always inhibits fluid flow from theintermediate portion 140i toward the front wheel brake cylinder 20. Thefluid flow between the intermediate portion 140i and the rear wheelbrake cylinder 30 are controlled by the second shut-off valve 140 only.In FIG. 22, reference sign 140n denotes a spring as biasing means forbiasing the valve member 140d in the direction away from the valve seat140f.

In the present check valve device 160, the central communication passage140g in which the first check valve 162 is provided, and the annularpassage 179 in which the second check valve 164 is provided are formedconcentrically with each other, whereby the overall dimension of thecheck valve device 160 in the radial or diametric direction of the valveseat member 140e can be reduced to minimize the size of the brakingsystem equipped with the check valve device 160.

Further, the valve seat member 140e, having the valve seats 140f and140m, also serves as a means for defining the communication passage 140gand annular passage 179, and further functions to support the first andsecond check valves 162, 164. This arrangement is effective to minimizethe number of the required components and the size of the braking systemas a whole.

It will be understood from the above description of the present secondembodiment that the check valve device 160, controller 170, and P valve110 cooperate to constitute an example of the distribution controldevice for controlling the distribution of the braking forces to beapplied to the front and rear wheels, according to a selected one of thefirst and second distribution patterns. It will also be understood thatthe passage 178 includes a portion which serves as a portion of the rearbrake cylinder passage 24 between the point of connection to the frontbrake cylinder passage 24 and the point of connect on to the pumppassage 148, and that the central communication passage 140g and theannular passage 179 serve respectively as two concentric and mutuallyindependent passages one of which has a circular cross section and theother of which has an annular cross section.

It is further noted that the P valve 110 is disposed outside a hydrauliccircuit which includes the first, second, and third shut-off valves 100,140, 146, reservoir 144 and pump 150. This arrangement makes it possibleto manufacture the P valve 110 separately from an integral brake unit,which includes the shut-off valves 100, 140, 146, reservoir 144, andpump 150 accommodated in a single common housing. Thus, the size andweight of the brake unit can be reduced. Where the P valve 110 and thebrake unit are manufactured separately from each other, it is a commonpractice to install the brake unit near the vehicle's engine, moreprecisely, near the master cylinder 10, and locate the P valve at aportion of the vehicle body where the a conduit providing the rear brakecylinder passage 24 is supported, or near the brake, which includes therear wheel brake cylinder 30.

Referring next to FIGS. 23-26(b), a third embodiment of the presentinvention will be described. The third embodiment is different from thesecond embodiment only in the manner in which the second shut-off valve140 is controlled by the controller 170.

In the fourth pressure control mode, the pressure in the front wheelbrake cylinder 20 is increased at a relatively slow rate while thepressure in the rear wheel brake cylinder 30 is increased at arelatively rapid rate (indicated in FIG. 20). In this fourth mode, therear wheel braking pressure is increased by operation of the pump 150.In this respect, the pump 150 does not deliver the pressurized brakefluid continuously but delivers the fluid intermittently, as indicatedin FIG. 26(a). Therefore, if the second shut-off valve 140 is held openin the fourth pressure control mode for a period longer than thedelivery period of the pump 150, the entire amount of the fluiddelivered by each delivery action of the pump 150 is supplied to therear wheel brake cylinder 30, unless the first check valve 162 is openedby the delivery pressure of the pump 150. On the other hand, thediameter of the rear wheel brake cylinder 30 is usually smaller thanthat of the front wheel brake cylinder 20. Accordingly, when the sameamount of brake fluid is supplied to the front and rear wheel brakecylinders 20, 30, the pressure in the rear wheel brake cylinder 30 ismore sensitively increased. Therefore, continuous control of the wheelbraking pressures in the fourth mode, with the second shut-off valve 140held open, will result in an excessively rapid increase in the rearwheel braking pressure. This will lead to a undesirable reduction in thecontrol stability of the rear wheel braking pressure due to an overshootof the pressure rise.

In the light of the above drawback, the braking system according to thepresent third embodiment has a duty-cycle pressure control mode in whichthe second shut-off valve 140 is alternately turned on and off with acontrolled duty cycle while the first and third shut-off valves 100, 146are held closed. This duty-cycle pressure control mode is considered tobe a compromise between the fourth mode (for slow increase of the frontwheel braking pressure and rapid increase of the rear wheel brakingpressure) and the fifth mode (for rapid increase of the front wheelbraking pressure and holding of the rear wheel braking pressure).

The duty cycle of the solenoid of the second shut-off valve 140 in theduty-cycle pressure control mode is not a fixed value but is variablefor continuous variation of the increase rates of the front and rearwheel braking pressures.

In the duty-cycle pressure control mode, the tendency to increase therear wheel braking pressure is increased if the characteristic of thefourth pressure control mode is dominant over that of the fifth pressurecontrol mode; while the tendency toward increasing the front wheelbraking pressure is increased if the characteristic of the fifth mode isdominant over that of the fourth mode. Therefore, the duty-cyclepressure control mode, wherein the duty cycle of the second shut-offvalve 140 can be continuously changed, provides a high desire ofstability without an excessively high rate of increase in the rear wheelbraking pressure. In addition, it assures adequate control of thedistribution of the front and rear wheel braking pressures, namely,adequate control of distribution of the braking forces to be applied tothe front and rear wheels.

In the present third embodiment, the ROM of the controller 170 storesroutines for controlling the second shut-off valve 140, as illustratedin the flow charts of FIGS. 23-25. The flow chart of FIG. 23 illustratesthe routine for controlling the solenoid of the second shut-off valve140. The flow chart of FIG. 24 illustrates a sub-routine executed instep S40 of the routine of FIG. 23. The flow chart of FIG. 25illustrates the routine for determining an OFF time T₁ of the solenoid.

These routines will be first briefly explained.

To assure an increase in the pressure in the rear wheel brake cylinder30 due to operation of the pump 150, it is necessary to open the secondshut-off valve 140 just when the pressurized brake fluid is deliveredfrom the pump 150 and supplied to the rear wheel brake cylinder 30. Inthe present embodiment, the second shut-off valve 140 is open while itssolenoid is held de-energized or OFF, namely, while the de-energizationpulse is present. However, it is difficult to generate thede-energization pulses in synchronization with the intermittent deliveryactions of the pump 150, respectively. The pump 150 has an operationcycle time consisting of the delivery time and the non-delivery time,which are substantially the same. The present embodiment is furtheradapted to allow pairs of adjacent de-energization pulses to begenerated with a cycle time T₃, as indicated in FIG. 26(b). Eachde-energization pulse has the width corresponding to the OFF time T₁ ofthe solenoid of the second shut-off valve 140 during which the valve 140is held open. The two adjacent de-energization pulses have an intervalT₂ which includes the solenoid OFF time T₁, (also indicated in FIG.26(b). This pulse interval T₂ is made equal to the delivery time of thepump 150, which is one half of the operation cycle time of the pump 150.According to this arrangement, the solenoid OFF time T₁ (open time ofthe shut-off valve 140) produced by one of the two de-energizationpulses of each pair is usually held within the delivery time of thecorresponding delivery time of the pump 150, even when the generation ofthe pair of de-energization pulses is not precisely timed with thedelivery action of the pump 150 in its intermittent delivery operation.In this example shown in FIG. 26(b), the entirety of the OFF time T₁ ofthe former de-energization pulse occurs within the correspondingdelivery time of the pump 150. However, the OFF times T₁ of the twode-energization pulses may partially overlap the corresponding deliverytimes of the pump 150. In addition, the total time during which thesecond shut-off valve 140 is open by the two pulses is equal to T₁.

Although it is possible to generate the de-energization pulse for eachdelivery action or time of the pump 150, this arrangement is notdesirable since the solenoid of the second shut-off valve 140 should beturned off and on for each delivery action of the pump 150, and thevalve 140 should have a high response to the generation of thede-energization pulses. Further, this arrangement tends to cause a rapidincrease in the rear wheel braking pressure. In view of these facts, thepresent embodiment utilizes pairs of de-energization pulses that aregenerated with the cycle time T₃, which is two times the operation cycletime of the pump 150. That is, each pair of de-energization pulses isgenerated each time the pump 150 performs two adjacent delivery actions.Accordingly, during the cycle time T₃, the pressurized brake fluiddelivered from the pump 150 is supplied to the front wheel brakecylinder 20 for a time period T₃ -2T₁, and is supplied to the rear wheelbrake cylinder 30 for a time period T₁. Therefore, the ratio of theamounts of the fluid supplied from the pump 150 to the front and rearwheel brake cylinders 20, 30 is proportional to (T₃ -2T₁)/T₁.

As the ratio (T₃ -2T₁)/T₁ increases, the rate of increase in the frontwheel braking pressure increases while the rate of increase in the rearwheel braking pressure decreases, whereby he braking force acting on thefront wheel increases while the braking source acting on the rear wheeldecreases. Thus, a relationship exits between the ratio of the amountsof the brake fluid supply from the pump 150 to the front and rear wheelbrake cylinders 20, 30, and the ratio of the braking pressures or forcesof the front and rear wheels. Namely, the ratios of the front and rearwheel braking pressures and forces increase with an increase in theratio of the amounts of the fluid supply from the pump 150 to the frontand rear wheel brake cylinders 20, 30.

Accordingly, the ratio of the braking forces of the front and rearwheels can be varied by changing the ratio of the amounts of the fluidsupply from the pump 150 to the front and rear wheel brake cylinders 20,30. The latter ratio can be changed by changing either the solenoid OFFtime T₁ (open time) of the second shut-off valve 140 or the cycle timeT₃ at which the successive pairs of de-energization pulses aregenerated, or both. If the solenoid OFF time T₁ is increased, forexample, the ratio of the amount of fluid supply to the front wheelbrake cylinder 20 to that to the rear wheel brake cylinder 30 isreduced, and the ratio of the braking force of the front wheel to thatof the rear wheel is accordingly reduced. If the solenoid OFF time T₁ isreduced, the ratio of the amount of fluid supply to the front wheelbrake cylinder 20 to that of the rear wheel brake cylinder 30 isincreased, and the ratio of the braking force of the front wheel to thatof the rear wheel is accordingly increased. If the cycle time T₃ isincreased, the ratio of the fluid supply amounts of the front and rearwheel brake cylinders 20, 30 is increased, and-the ratio of the brakingforces of the front and rear wheels is accordingly increased. If thecycle time T₃ is reduced, the ratios of the fluid supply amounts of thefront and rear wheel brake cylinders 20, 30 and the braking forces ofthe front and rear wheels are reduced.

In the present third embodiment, only the solenoid OFF time T₁ of thesecond shut-off valve 140 is increased or reduced to continuously changethe duty cycle of the shut-off valve 140, which is the ratio of the opentime to the closed time of the shut-off valve 140.

Further, in the present embodiment, the solenoid OFF time T₁ isdetermined on the basis of the numbers of reductions and increases ofthe front and rear wheel braking pressures. To this end, the controller170 is provided with a pressure reduction counter CFR which isincremented when the front wheel braking pressure is reduced once, anddecremented when the rear wheel braking pressure is reduced once. Thecontent of this pressure reduction counter CFR indicates a relationshipbetween the pressure reduction frequencies of the front and rear wheelbrake cylinders 20, 30. The solenoid OFF time T₁ (width of eachde-energization pulse) is increased by a predetermined constant value αeach time the counter CFR value exceeds a positive threshold value +K,and decreased by the value α each time the counter CFR value becomessmaller than a negative threshold value -K. The OFF time T₁ is variablewithin a range between 0 and T₃.

If the reduction of the front wheel braking pressure is relativelyfrequent (if the front wheel brake cylinder 20 has exhibited arelatively high tendency of pressure reduction), the CFR value exceedsthe positive threshold value +K, and the OFF time or open time T₁ of thesecond shut-off valve 140 is increased, whereby the amount of fluidsupply from the pump 150 to the front wheel brake cylinder 20 is reducedto reduce the rate of increase in the front wheel braking pressure whichthereby reduces the braking force applied to the front wheel. On theother hand, an increase in the amount of the fluid supply from the pump150 to the rear wheel brake cylinder 30 increases the rate of increasein the rear wheel braking pressure, which corresponds to increasedbraking force applied to the rear wheel. In this case, therefore, thebraking function of the front wheel is reduced while that of the rearwheel is increased.

If the reduction of the rear wheel braking pressure is relatively theCFR value becomes smaller than the negative threshold value -K, and theOFF time or open time T₁ of the second shut-off valve 140 is reduced,whereby an increase in the amount of the fluid supply from the pump 150to the front wheel brake cylinder 20 increases the rate of increase inthe front wheel braking pressure, which corresponds to braking forceapplied to the font wheel. On the other hand, the fluid supply from thepump 150 to the rear wheel brake cylinder 30 is inhibited to hold therear wheel braking pressure at the present level for thereby maintainingthe braking force presently acting on the rear wheel. In this case,therefore, the braking function of the front wheel is increased while anincrease in the braking function of the rear wheel is inhibited.

It will be understood that the present third embodiment is adapted suchthat the duty cycle of the solenoid of the third shut-off valve 140 ischanged on the basis of at least one of the pressure reducing tendencies(pressure reducing hystereses) of the front and rear wheel brakecylinders 20, 30, by changing the solenoid OFF time T₁ (width of thede-energization pulse of the second shut-off valve 140) on the basis ofthe content of the pressure reducing counter CFR.

Referring to the flow charts of FIGS. 23 and 24, there will next bedescribed in detail the routine for controlling the second shut-offvalve 140. In this routine, the second shut-off valve 140 is controllednot only in the duty-cycle mode but also in the other pressure controlmodes, as explained below.

The present routine of FIG. 23 for controlling the second shut-off valve140 is executed at a predetermined time interval. The routine isinitiated with step S10 to determine whether the braking system is inthe process of the anti-lock pressure control. This is determined basedon the flags provided in the RAM of the controller 170. If a negativedecision (NO) is obtained in step S10, the control flow goes to step S60in which a signal is generated to de-energized or turn OFF the solenoidof the second shut-off valve 140, to hold the valve 140 in the openstate. Thus, one cycle of the present routine is terminated.

If the anti-lock pressure control of the braking system is commencedduring repetitive execution of the routine, an affirmative decision(YES) is obtained in step S10, and the control flow goes to step S20 todetermine whether it is necessary to reduce the front wheel brakingpressure. This determination is made based on a flag stored in the RAM.If an affirmative decision (YES) is obtained step S20, the control flowgoes to step S60 to turn OFF the solenoid of the second shut-off valve140 for opening the valve 140. To reduce the front wheel brakingpressure, the second shut-off valve 140 should be opened in the seventhpressure control mode as described above. In this seventh mode, thepressures in both of the front and rear wheel brake cylinders 20, 30 arereduced.

If a negative decision (NO) is obtained in step S20, the control flowgoes to step S30 to determine whether it is necessary to reduce the rearwheel braking pressure. This step S30 is provided to determine whetherthe pressure reduction is required for only the rear wheel brakecylinder 30. If an affirmative decision (YES) is obtained in step S30,the control flow goes to step S50 to energize or turn ON the solenoid ofthe shut-off valve 140 for closing the valve 140. In this case, only therear wheel braking pressure is reduced regardless of the front wheelbraking pressure.

If no pressure reduction is required for not only the front wheel brakecylinder 20 but also the rear wheel brake cylinder 30, a negativedecision (NO) is obtained in step S30, and step S40 is implemented tocontrol the second shut-off valve 140 in the duty-cycle pressure controlmode. In the preceding embodiment, the fourth or fifth pressure controlmode is selected in this situation. In the present embodiment, theduty-cycle pressure control mode is selected rather than the fourth orfifth mode.

In theory, the duty-cycle pressure control mode of operation is alsoperformed in the first or second pressure control mode indicated inTABLE 1. In practice, however, the first and second modes are rarelyselected during the anti-lock pressure control operation. In this sense,the duty-cycle pressure control mode is used as an alternative to thefourth or fifth mode.

The sub-routine for controlling the second shut-off valve 140 in theduty-cycle pressure control mode is illustrated in detail in the flowchart of FIG. 24. In this sub-routine, step S100 is initiallyimplemented to: read out the cycle time T₃ (which is a predeterminedconstant) from the ROM of the controller 170; read out from the RAM ofthe controller 170 a time lapse T from the start of the present cycle inwhich a pair of de-energization pulses each defining the solenoid OFFtime T₁ is generated; and determine whether the time lapse T has reachedthe predetermined cycle time T₃. Namely, step S100 is provided todetermine whether the predetermined cycle time T₃ has passed. If anegative decision (NO) is obtained in step S100, step S110 isimplemented to: read out the presently effective solenoid OFF time T₁(from the RAM; and determine whether the time lapse T is shorter thanthe solenoid OFF time T₁. The solenoid OFF time T₁ is determined by aroutine illustrated in the flow of FIG. 25 as described below, andstored in the RAM. If an affirmative decision (YES) is obtained in stepS110, the control flow goes to step S120 to turn OFF the solenoid of theshut-off valve 140 for opening the valve 140. Namely, the first one ofthe pair of de-energization pulses in question is generated. Then, stepS130 is implemented to increment the time lapse T by a predeterminedvalue ΔT. Thus, one cycle of execution of the sub-routine of FIG. 24 isterminated, and the control flow goes back to the main routine of FIG.23. The sub-routine of FIG. 24 is executed each time step S40 of themain routine of FIG. 23 is executed. The following description refers tothe situation where step S40 of the main routine of FIG. 23 or thesub-routine of FIG. 24 is repeatedly implemented without implementationof steps S50 and S60.

When the time lapse T has reached the solenoid OFF time T₁ as a resultof repeated execution of the sub-routine of FIG. 24, an affirmativedecision (YES) is obtained in step Sl10, and the control flow goes tostep S150 to read out the predetermined pulse interval T₂ from the ROMof the controller 170, and determine whether the time lapse T is shorterthan the pulse interval T₂. If an affirmative decision (YES) is obtainedin step S150, the control flow goes to step S160 in which the solenoidof the second shut-off valve 140 is turned ON to close the valve 140.That is, the first de-energization pulse is terminated and replaced byan energization pulse which energizes or turn ON the solenoid. Step S160is followed by step S130.

If the time lapse T has reached pulse interval T₂ during the repeatedexecution of the sub-routine of FIG. 24, a negative decision (NO) isobtained in step S150, and the control flow goes to step S170 todetermine whether the time lapse T is shorter than a sum of the solenoidOFF time T₁ and the pulse interval T₂. If an affirmative decision (YES)is obtained in step S170, step S180 is implemented to turn OFF thesolenoid of the shut-off valve 140 to open the valve 140. Namely, thesecond one of the pair of de-energization pulses in question isgenerated. Then, the control flow goes to step S130.

If the time lapse T has reached the sum of (T₁ +T₂) during the repeatedexecution of the sub-routine of FIG. 24, a negative decision (NO) isobtained in step S170, and the control flow goes to step S190 to turn ONthe solenoid of the shut-off valve 140 to close the valve 140. Thus, thesecond de-energization pulse is terminated. Then, the control flow goesto step S130.

When the time lapse T increases to the predetermined cycle time T₃ as aresult of the repeated execution of the sub-routine of FIG. 24, anaffirmative decision (YES) is obtained in step S100, and step S140 isimplemented to reset the time lapse for starting the next cycle ofgeneration of a pair of de-energization pulses.

With the sub-routine of FIG. 24 executed repeatedly, the second shut-offvalve 140 is controlled (opened and closed) by successive pairs ofde-energization pulses which are generated periodically with the cycletime T₃, such that the two de-energization pulses of each pair have thepulse interval T₂, and each pulse has a width corresponding to thesolenoid OFF time T₁ of the second shut-off valve 140, as indicated inFIG. 26(b).

The duty cycle of the second shut-off valve 140 is controlled bydetermining the solenoid OFF time T₁ according to the routine of FIG.25, which will be described in detail.

The routine of FIG. 25 is executed at a predetermined interval. Theroutine is initiated with step S200 to determine whether the brakingsystem is in the of the anti-lock pressure control. If a negativedecision (NO) is obtained in step S200, the control flow goes to stepS210 to reset the content of the pressure reduction counter CFR andflags FX and FY and also rest the solenoid OFF time T₁ to apredetermined initial value T₁₀ The content of the counter CFR, thevalues of the flags FX, FY and the OFF time T₁ are stored in the RAM ofthe controller 170. The functions of the flags FX, FY will be described.One cycle of execution of the routine of FIG. 25 is terminated with stepS210.

If the anti-lock pressure control is commenced during repeated executionof the routine of FIG. 25, an affirmative decision (YES) is obtained instep S200, and the control flow goes to step S220 to determine whetherit is required to reduce the front wheel braking pressure. If anaffirmative decision (YES) is obtained in step S220, step S230 isimplemented to determine whether the flag FX is set at "0". If anaffirmative decision (YES) is obtained in step S230, the control flowgoes to step S240 to increment the pressure reduction counter CFR, andthen step S240 the flag FX to "1".

Step S250 is followed by step S260 to determine whether the content ofthe counter CFR is larger than the positive threshold value +K. If anaffirmative decision (YES) is obtained in step S260, step S310 isimplemented to increase the solenoid OFF time T₁ by a predeterminedvalue a, and store the increased OFF time T₁ in the RAM. Step S310 isfollowed by step S320 to reset the pressure reduction counter CFR tozero.

Then, step S270 is implemented to determine whether the content of thecounter CFR is smaller than the negative threshold value -K. If anegative decision (NO) is obtained in step S270, the present cycle ofexecution of the routine of FIG. 25 is terminated.

If reduction of the front wheel braking pressure is still required inthe next cycle, an affirmative decision (YES) is obtained in step S220,and a negative decision (NO) is obtained in step S230, since the flag FXhas been set at "1". Consequently, the control flow goes to step S260while skipping steps S240 and S250. Thus, the flag FX is provided toincrement the pressure reduction counter CFR only once during repeatedexecution of the present routine while the reduction of the front wheelbraking pressure is continuously required. In other words, the flag FXfunctions to increment the counter CFR only once for generation of eachcommand to reduce the front wheel braking pressure. If the reduction ofthe front wheel braking pressure is no more required, a negativedecision (NO) is obtained in step S220, and the control flow goes tostep S280 to reset the flag FX to zero. Then, step S290 and thefollowing steps S300, S330, S340 and S350 are implemented fordecrementing the counter CFR. Steps S290, S300, S330, S340 and S350 forthe rear wheel brake cylinder 30 are equivalent to steps S220, S280,S230, S240 and S250 for the front wheel brake cylinder 20.

Then, step S260 is implemented to determine whether the content of thecounter CFR is larger than the positive threshold value +K. If anegative decision (NO) is obtained in step S260, step S270 isimplemented to determine whether the content of the counter CFR issmaller than the negative threshold value -K. If a negative decision(NO) is obtained in step S270, the present cycle of execution of theroutine of FIG. 25 is terminated. If an affirmative decision (YES) isobtained in step S270, the control flow goes to step S360 to read outthe solenoid OFF time T₁ from the RAM of the controller 170, and reducethe solenoid OFF time T₁ by the predetermined value a. Step S360 isfollowed by step S370 in which the counter CFR is reset to zero, and onecycle of execution of the routine of FIG. 25 is terminated.

It will be understood from the above explanation of the present thirdembodiment that the check valve device 160 functions as a pressurereduction control device, which cooperates with the controller 170 and Pvalve 100 to comprise the distribution control device for controllingthe distribution of the braking forces of the front and rear wheels,according to one of the first and second distribution patterns. It willalso be understood that the portions of the controller 170 assigned toimplement step S40 of the routine of FIG. 23 (namely, sub-routine ofFIG. 24 for controlling the second shut-off valve 140 in the duty-cyclepressure control mode) and the routine of FIG. 25 for determining thesolenoid OFF time T₁ function as means for changing the duty cycle ofthe shut-off valve 140 on the basis of the numbers of the pressurereductions which have been required for the wheel brake cylinders 20,30.

Referring next to FIGS. 27 and 28, there will be described a fourthembodiment of the present invention.

The present fourth embodiment is different from the second embodimentonly in the construction of the P valve and the connection of the Pvalve to the pump 150. In the second embodiment of FIG. 17, the outputor delivery end of the pump passage 148 is connected to a portion of therear brake cylinder passage 24 which is upstream of the second shut-offvalve 140 and the P valve 110, so that the brake fluid delivered fromthe pump 150 is supplied to the rear wheel brake cylinder 30 through theshut-off valve 140 and the P valve 110. In this arrangement, the brakingpressure in the rear wheel brake cylinder 30 is influenced by thepressure reducing function of the P valve 110 even during the anti-lockpressure control operation. This arrangement does not permit the rearwheel braking pressure to be increased to a sufficiently high level. Inview of this drawback, the present fourth embodiment uses a P valve 180which is directly connected to the pump 150, as shown in FIG. 28.

The construction of the P valve 180 will be described by reference toFIG. 28. Since the P valve 180 is basically similar to the P valve 110,only a difference of the P valve 180 from the P valve 110 will bedescribed. The same reference numerals as used in FIG. 18 will be usedto identify the corresponding components of the P valve 180, which willnot be described in the interest of simplification.

The P valve 180 is designed so that it does not function during theanti-lock pressure control operation. Namely, the P valve 180 is adaptedto receive the delivery pressure of the pump 150 during the anti-lockpressure control, which inhibits the valve piston 124 from performing apressure reducing function, that is, prevents the valve piston 124 frombeing seated on the cup seal 128.

Described in detail, the small-diameter portion 122 of the valve piston124 is not directly exposed to an air chamber 181, but an auxiliarypiston 182 fixed to the small-diameter portion 122 is exposed to the airchamber 181. A two-way sealing member in the form of an O-ring 183 formsa fluid-tight fit with the small-diameter portion 122, while a one-waysealing member in the form of a cup seal 184 forms a fluid-tight fitwith the auxiliary piston 182. The 0-ring 183 and the cup seal 184define a pump pressure chamber 185 therebetween. The pump pressurechamber 185 is connected to the delivery or output side of the pump 150through a communication passage 187 formed through a plug 186, acommunication passage 188 formed through the housing 112, and a fluidpassage 189, which is connected to the pump passage 148 as shown in FIG.28.

The auxiliary piston 182 has a center blind hole 190 open on its endface and exposed to the air chamber 181, such that the blind hole 190forms a part of the air chamber 181. In other words, the blind hole 190is formed to increase the volume of the air chamber 181, so that thevolume of the air chamber 181 can be easily reduced when the valvepiston 124 operates to perform its pressure reducing function. Since thesame valve piston 124 as used in the P valve of FIG. 18 is used in thepresent P valve 180, the small-diameter portion 122 has a central blindhole 191, but this blind hole 191 is not essential in the presentembodiment.

The auxiliary piston 182 is normally held in abutting contact with thevalve piston 124 by a biasing means in the form of a spring 192. Theopening pressure difference of the P valve 180 is determined by thebiasing forces of both of the springs 126 and 192.

The 0-ring 183 functions to inhibit not only a flow of the brake fluidin a direction from the input chamber 130 toward the pump pressurechamber 185, but also a flow of the brake fluid n the oppositedirection.

In the anti-lock braking system provided with the P valve 180constructed as described above, the brake fluid delivered from the pump150 to increase the rear wheel braking pressure during the anti-lockpressure control is supplied to the rear wheel brake cylinder 30 throughthe second shut-off valve 140 and the P valve 180. At this time, the Pvalve 180 does not function to reduce the pressure of the fluid receivedfrom the pump 150. Explained more particularly, the auxiliary piston 182is moved in a direction (in the right direction as seen in FIG. 28) thatcauses reduction in the volume of the air chamber 181, whereby theauxiliary piston 182 is brought into contact with the bottom of the airchamber 181. At the same time, the valve piston 124 is moved in theopposite direction (in the left direction as seen in FIG. 28) so thatthe large-diameter portion 120 is brought into contact with the bottomof the housing 112. Although the shoulder surface of the bottomed valvepiston 124 is in fluid-tight contact with the cup seal 128, theprotrusions provided on the cup seal 128 as described above with respectto the P valve 110 permit the flow of the fluid between the input andoutput chambers 130, 132. As a result, the P valve 180 simply functionsas part of the rear brake cylinder passage Therefore, the brakingpressure in the rear wheel brake cylinder 30 can be increased to a levelat which the check valve 156 is opened, namely, to the pressure in themaster cylinder 10. Thus, the rear wheel braking pressure can beincreased to a sufficiently high level, and the friction coefficient ofthe road surface on which the rear wheel lies can be effectively used tobrake the vehicle, even in the presence of the P valve 180.

Referring to the graph of FIG. 29, there will be described anadvantageous effect of the present fourth embodiment of the invention.

When the depression force acting on the brake pedal 14 is increased fromzero in a normal braking condition without an anti-lock pressurecontrol, the front-rear force distribution point is moved from the zeropoint of the coordinate system of the graph of FIG. 29, along a firstbasic distribution line (which is determined by the basic brakingarrangement without the shut-off valves 100, 140, 146 and the firstcheck valve 162). If the depression force on the brake pedal 14 isfurther increased during the minimum-load run of the vehicle, the forcedistribution point is further moved to point "a" of intersection betweena distribution line of the P valve 180 and a front wheel locking linefor the minimum-load run, as indicated in FIG. 29.

If anti-lock control is commenced in this condition to reduce both ofthe front and rear wheel braking pressures in the seventh pressurecontrol mode, the force distribution point is moved to point "b'". Ifthe fourth pressure control mode is established to reduce the front andrear wheel braking pressures as a result of elimination of the lockingtendency of the front wheel, the front wheel braking pressure is heldconstant until the first check valve 172 is opened, and is thenincreased. Accordingly, the force distribution point is moved from point"b" in a direction parallel to the vertical axis of the graph of FIG.29, which direction causes an increase in the rear wheel braking force.Since the P valve 180 is disabled while the rear wheel braking force isincreased by operation of the pump 150, the force distribution point isthen moved to point "c" which lies on a rear wheel locking line for theminimum-load run. As a result, the anti-lock control of the rear wheelbraking pressure is initiated to eliminate the locking tendency of therear wheel.

If the force distribution point is moved to point "d", indicated in FIG.29, during a normal braking operation with the vehicle placed in thefull-load run, the anti-lock control of the front wheel braking pressureis initiated, and the force distribution point is then moved to point"b" as a result of the anti-lock pressure control. If the lockingtendency of the front wheel is subsequently eliminated, the forcedistribution point is moved from point "b", via point "c", to point "e"lying on a second basic distribution line, since the P valve 180 isdisabled. The second basic distribution line is determined by the basicbraking arrangement which includes the check valve device 160 (firstcheck valve 162) but does not include the P valve 180. Then, the forcedistribution point is moved along the second basic line, in a directionthat causes the rear wheel braking pressure to increase. Eventually, theforce distribution line reaches point "f", the intersection between thesecond basic distribution line and a front wheel locking line for thefull-load run. Then, the front wheel braking pressure is controlled inthe anti-lock manner to avoid the locking tendency of the front wheel.

In the present fourth embodiment, the actual distribution of the frontand rear wheel braking forces is controlled, during the normal pressurecontrol operation (without the anti-lock control of the brakingpressures), according to the distribution line of the P valve 180,irrespective of whether the vehicle is in the minimum-load run orfull-load run. During the anti-lock pressure control operation with thevehicle in the minimum-load run, on the other hand, the actualdistribution of the front and rear wheel braking forces is controlledaccording to a distribution pattern which is sufficiently similar andclose to the ideal distribution curve for the minimum-load run. Duringthe anti-lock pressure control operation with the vehicle in thefull-load run, the actual distribution of the front and rear wheelbraking forces controlled according to a portion of the idealdistribution curve for the full-load run which is located below (in FIG.29) the second basic distribution line, and a portion of the secondbasic distribution line which is located below the ideal distributioncurve for the full-load run. In the present embodiment, therefore, therear wheel braking pressure during the full-load vehicle run can befurther increased, whereby the total braking force of the vehicle can beincreased, and the required vehicle braking distance can be reduced. Itwill also be understood from the graph of FIG. 29 that the rear wheelbraking pressure or force during the full-load vehicle run can beeffectively increased to thereby reduce the required vehicle brakingdistance, even when the ideal rear braking force is smaller than thethreshold level of the P valve 180.

In the present embodiment, the distribution of the front and rear wheelbraking forces during normal pressure control operation and anti-lockpressure control operation in combination with the minimum-load vehiclerun correspond to the first distribution pattern, while the distributionof the front and rear wheel braking forces during the anti-lock pressurecontrol operation in combination with the full-load vehicle runcorresponds to the second distribution pattern. The second distributionpattern defines the rear wheel braking force larger than that defined bythe first distribution pattern, over the entire range of the rear wheelbraking force.

Like the braking system according to the second embodiment, theanti-lock braking system according to the fourth embodiment is capableof reducing the required vehicle braking distance during the full-loadrun, not only during braking with a relatively large braking force on arelatively high friction-coefficient road surface, but also duringbraking with a relatively small braking force on a relatively lowfriction-coefficient road surface. As indicated above with respect tothe second embodiment, the required braking distance during thefull-load vehicle run with a relatively large braking force cannotalways be effectively reduced, but the required braking distance duringthe full-load vehicle run with a relatively small braking force canalways be effectively reduced.

In the present fourth embodiment, the check valve device 160 functionsas the pressure reduction control device, which cooperates with thecontroller 170 and P valve 180 to constitute the distribution controldevice for controlling the distribution of the braking forces of thefront and rear wheels, according to a selected one of the first andsecond distribution patterns. Further, the portions of the controller170 assigned to implement step S40 of FIG. 23 (sub-routine of FIG. 24)and the routine of FIG. 25 function as the means for changing the dutycycle of the shut-off valve 140 on the basis of the numbers of thepressure reductions which have been required for the front and rearwheel brake cylinders.

A fifth embodiment of this invention will be described by reference toFIG. 30.

Like the anti-lock braking system according to the preceding fourthembodiment, the anti-lock braking system according to the presentembodiment is also designed to be able to increase the rear wheelbraking pressure to the level of the master cylinder pressure during theanti-lock pressure control, even in the presence of the P valve. In thefourth embodiment of FIG. 27, the P valve 180 is located between therear wheel brake cylinder 30 and the second shut-off valve 140 as in thesecond embodiment of FIG. 17, but the P valve 180 is provided with meansfor disabling its pressure reducing function. In the present embodimentof FIG. 30, the P valve 110 as used in the second embodiment is used,but this P valve 110 is located upstream of the check valve device 160.

Described in detail, the P valve 110 constructed as shown in FIG. 18 isdisposed in a portion of the rear brake cylinder passage 24 of the pointof connection between the front and rear brake cylinder passages 22, 24and the check valve device 160. Thus, the delivery or output end of thepump passage 148 is located downstream of the P valve 110, so that thebrake fluid delivered from the pump 150 during the anti-lock pressurecontrol is supplied to the rear wheel brake cylinder 30 without passingthrough the P valve 110. Accordingly, the rear wheel braking pressurewhich is increased by operation of the pump 150 is not influenced by thepressure reducing function of the P valve 110. In the present fifthembodiment in which the P valve 110 is disposed between the front wheelbrake cylinder 20 and the pump 150, the brake fluid delivered from thepump 150 during the anti-lock pressure control is first supplied to theoutput chamber 132 of the P valve 110 as shown in FIG. 18. As a result,the output pressure of the P valve 110 is increased, and the valvepiston 124 is brought into abutting contact with the bottom of the plug193, which constitutes a part of the housing 112. Subsequently, only theoutput pressure of the P valve is increased while the input pressure isheld constant. When the output pressure of the P valve 110 exceeds theinput pressure, the one-way sealing portion 134 of the cup seal 128 isopened, and the brake fluid is permitted to flow in the direction fromthe output chamber 132 toward the input chamber 130, through a gapbetween the one-way sealing portion 134 and the cylinder bore 118,whereby the brake fluid delivered from the pump 150 is supplied to thefront wheel brake cylinder 20 through the P valve 110. Thus, the frontwheel braking pressure is increased by operation of the pump 150.

In the present fifth embodiment, the check valve device 160 functions asthe pressure reduction control device, which cooperates with thecontroller 170 and P valve 110 to constitute the distribution controldevice for controlling the distribution of the braking forces of thefront and rear wheels, according to a selected one of the first andsecond distribution patterns. Further, the portions of the controller170 assigned to implement the sub-routine of FIG. 24 and the routine ofFIG. 25 function as the means for changing the duty cycle of theshut-off valve 140 on the basis of the observed number of the pressurereductions which have been required for the front and rear wheel brakecylinders.

In the detailed descriptions of the several embodiments, the openingpressure difference of the first check valve 72, 162 is fixed. However,the opening pressure difference may be variable to allow the openingpressure difference to increase with an increase in the vehicle load orin the friction coefficient of the road surface.

While the preferred embodiments of the present invention have beendescribed in detail by reference to the accompanying drawings, it shouldbe understood that the present invention may also exists with variouschanges, modifications, and improvements, which may occur to thoseskilled in the art, without departing from the scope of the inventiondefined in the following claims.

What is claimed is:
 1. A braking system for braking a motor vehicle byoperation of a front and a rear brake for a front and a rear wheel ofthe vehicle, respectively, said braking system comprising:a distributioncontrol device to control a distribution of a front wheel braking forceand a rear wheel braking force which are produced by said front and rearbrakes, respectively, and which are applied to said front and rearwheels, said distribution control device controlling said distributionaccording to a selected one of a first distribution pattern and a seconddistribution pattern, each of said first and second distributin patternsbeing a relationship between said front and rear wheel braking forcessuch that said rear wheel braking force defined by said seconddistribution pattern is larger than that defined by said firstdistribution pattern at least when the front and rear wheel brakingforces are smaller than respective predetermined values, and when atleast said rear wheel braking force is being increased.
 2. A brakingsystem according to claim 1, further comprising a hydraulic pressuresource for pressuring a working fluid, and wherein said front and rearbrakes include, respectively, a front and rear wheel brake cylinderwhich are supplied with the working fluid pressurized by said hydraulicpressure source, and wherein said distribution control devicecomprises:(a) a check valve disposed between said front wheel brakecylinder and said hydraulic pressure source, said check valve permittinga flow of the fluid in a first direction from said pressure sourcetowards said front wheel brake cylinder after the pressure generated bysaid pressure source becomes higher than the pressure in said frontwheel brake cylinder by more than a predetermined difference, wherebythe pressure of the fluid to be applied to said front wheel brakecylinder through said check valve is reduced with respect to thepressure generated by said pressure source, said check valve inhibitinga flow of the fluid in a second direction opposite to said firstdirection; and (b) selective disabling means for selectively disablingsaid check valve to prevent functioning of said check valve to reducethe pressure to be applied to said front wheel brake cylinder.
 3. Abraking system according to claim 1, wherein said distribution controldevice controls said distribution of said front and rear wheel brakingforces according to said first distribution pattern when a load actingon the vehicle is smaller than a predetermined load value, and accordingto said second distribution pattern when said load is not smaller thansaid predetermined load value.
 4. A braking system according to claim 1of a diagonal or X-crossing type for a four-wheel motor vehicle, havingtwo pressure application sub-systems which are respectively connected totwo mutually independent pressurizing chambers of a master cylinder,each of said two pressure application sub-systems including:(a) a frontbrake cylinder passage connecting a corresponding one of said twopressurizing chambers of said master cylinder and a front wheel brakecylinder of said front brake, (b) a rear brake cylinder passageconnecting said front brake cylinder passage and a rear wheel brakecylinder of said rear brake, (c) a master cylinder cut valve in the formof a normally-open shut-off valve disposed in a portion of said frontbrake cylinder passage between said master cylinder and a point ofconnection of said front and rear brake cylinder passages, (d) areservoir passage connected at one of opposite ends thereof to said rearbrake cylinder passage, (e) a reservoir connected to the other end ofsaid reservoir passage, (f) a pressure reducing valve in the form of anormally-closed shut-off valve disposed in said reservoir passage, (g) apump passage connected at one of opposite ends thereof to said reservoirand at the other end to at least one of said front and rear brakecylinder passages, (h) a pump disposed in said pump passage fordelivering a working fluid from said reservoir to a portion of said eachpressure application sub-system, and (i) a controller operable in ananti-lock pressure control mode for controlling said master cylinder cutvalve, said pressure reducing valve and said pump to effect andanti-lock pressure control operation for controlling pressures of saidfluid in said front and rear wheel brake cylinders in anti-lock manner,and whereinsaid distribution control device comprises said controller,and a pressure reduction control device disposed in a portion of saideach pressure application sub-system which is other than the portionbetween said master cylinder and the connection of said front and rearbrake cylinder passages, said pressure reduction control device applyingto said front wheel brake cylinder the pressure as generated by a firsthydraulic pressure source in the form of said master cylinder, tothereby establish said first distribution pattern, when said controlleris not placed in said anti-lock pressure control mode, said controllerand said pressure reduction control device cooperating to establish saidsecond distribution pattern such that the pressure generated by a secondhydraulic pressure source which consists of at least one of said mastercylinder and said pump is reduced by said pressure reduction controldevice and is then applied to said front wheel brake cylinder, when saidcontroller is placed in said anti-lock pressure control mode.
 5. Abraking system according to claim 1 of a diagonal or X-crossing type fora four-wheel motor vehicle, having two pressure application sub-systemswhich are respectively connected to two mutually independentpressurizing chambers of a master cylinder, each of said two pressureapplication sub-systems including:(a) a front brake cylinder passageconnecting a corresponding one of said two pressurizing chambers of saidmaster cylinder and a front wheel brake cylinder of said front brake,(b) a rear brake cylinder passage connecting said front brake cylinderpassage and a rear wheel brake cylinder of said rear brake, (c) a mastercylinder cut valve in the form of a normally-open shut-off valvedisposed in a portion of said front brake cylinder passage between saidmaster cylinder and a point of connection of said front and rear brakecylinder passages, said master cylinder cut valve being closed when thebraking system is in an anti-lock pressure control mode, and opened whenthe braking system is not in said anti-lock pressure control mode, (d)an intermediate valve in the form of a normally-open shut-off valvedisposed in said rear brake cylinder passage, (e) a reservoir passageconnected at one of opposite ends thereof to a portion of said rearbrake cylinder passage between said intermediate valve and said rearwheel brake cylinder, (f) a reservoir connected to the other end of saidreservoir passage, (g) a pressure reducing valve in the form of anormally-closed shut-off valve disposed in said reservoir passage, (h) apump passage connected at one of opposite ends thereof to said reservoirand at the other end to a portion of said rear brake cylinder passagebetween said intermediate valve and a point of connection of said frontand rear brake cylinder passages, (i) a pump disposed in said pumppassage for delivering a working fluid from said reservoir to a portionof said each pressure application sub-system, and (j) a controlleroperable in an anti-lock pressure control mode for controlling saidmaster cylinder cut valve, said intermediate valve, said pressurereducing valve and said pump to effect an anti-lock pressure controloperation for controlling pressures of said fluid in said front and rearwheel brake cylinders in an anti-lock manner, and whereinsaiddistribution control device comprises said controller, and a check valvedevice disposed in a portion of said rear brake cylinder passage betweenthe point of connection of said front and rear brake cylinder passagesand a point of connection of said rear brake cylinder passage and saidpump passage, said check valve device comprising a first check valve anda second check valve, said first check valve permitting a flow of thefluid therethrough in a first direction from said pump toward said frontwheel brake cylinder after the pressure generated by the pump becomeshigher than the pressure in said front wheel brake cylinder by more thana predetermined difference, and inhibiting a flow of the fluidtherethrough in a second direction opposite to said first direction,said second check valve permitting a flow of the fluid therethrough insaid second direction and permitting a flow of the fluid therethrough insaid first direction.
 6. A braking system according to claim wherein atleast a part of said portion of said rear brake cylinder passage betweenthe point of connection of said front and rear brake cylinder passagesand the point of connection of said rear brake cylinder passage and saidpump passage consists of a first and a second passage which areconcentric with and mutually independent of each other and which have acircular and an annular cross sectional shape, respectively, said firstcheck valve being disposed in one of said first and second passageswhile said second check valve being disposed in the other of said firstand second passages.
 7. A braking system according to claim 5, whereinsaid distribution control device further comprises:(a) a proportioningvalve disposed in said rear brake cylinder passage and including a valvepiston which is operated to apply to said rear brake cylinder thepressure as generated by said master cylinder or said pump as ahydraulic pressure source when the pressure as generated by saidhydraulic pressure source is lower than a predetermined threshold level,and to reduce the pressure as generated by said hydraulic pressuresource and apply the reduced pressure to said rear wheel brake cylinderafter the pressure as generated by said hydraulic pressure sourceexceeds said predetermined threshold level, and (b) a disabling devicefor disabling said proportioning valve to prevent functioning of saidproportioning valve to reduce the pressure to be applied to said rearwheel brake cylinder when the pressure in said rear wheel brake cylinderis increased.
 8. A braking system according to claim 7, wherein saiddisabling device comprises means for applying the pressure generated bysaid pump to said valve piston of said proportioning valve in adirection opposite to a direction in which the valve piston is moved toreduce the pressure generated by said pump, whereby the proportioningvalve is disabled when the pressure in said rear wheel brake cylinder isincreased.
 9. A braking system according to claim 5, wherein saidcontroller has a plurality of pressure control modes which areselectively established to control said master cylinder cut valve, saidintermediate valve and said pressure reducing valve in said anti-lockmanner, said plurality of pressure control modes including:(1) a mode inwhich said intermediate valve and said pressure reducing valve are bothopen while said master cylinder cut valve is closed, to reduce thepressures in both of said front and rear wheel brake cylinders, (2) amode in which said master cylinder cut valve and said intermediate valveare both closed while said pressure reducing valve is open, to increasethe pressure in said front wheel brake cylinder by operation of saidpump, and reduce the pressure in said rear wheel brake cylinder, and (3)a duty-cycle pressure control mode in which said master cylinder cutvalve and said pressure reducing valves are both closed while saidintermediate valve is alternately closed and opened to increase thepressures in said front and rear wheel brake cylinders by operation ofsaid pump.
 10. A braking system according to claim 9, wherein saidcontroller comprises means for changing a duty cycle of the intermediatevalve in said duty-cycle pressure control mode.
 11. A braking systemaccording to claim 10, wherein said means for changing the duty cycle ofsaid intermediate valve changes said duty cycle on the basis of at leastone of pressure reducing tendency of said front wheel brake cylinder andpressure reducing tendency of said rear wheel brake cylinder, whichtendencies have been exhibited in said anti-lock pressure controloperation under the control of said controller.
 12. A braking systemaccording to claim 1, further comprising a hydraulic pressure source forpressuring a working fluid, and wherein said front and rear brakesinclude, respectively, a front and rear wheel brake cylinder which aresupplied with the working fluid pressurized by said hydraulic pressuresource, and wherein said distribution control device comprises:(a) acheck valve disposed between said front wheel brake cylinder and saidhydraulic pressure source, said check valve permitting a flow of thefluid in a first direction from said pressure source towards said frontwheel brake cylinder after the pressure generated by said pressuresource becomes higher than the pressure in said front wheel brakecylinder by more than a predetermined difference, whereby the pressureof the fluid to be applied to said front wheel brake cylinder throughsaid check valve is reduced with respect to the pressure generated bysaid pressure source, said check valve inhibiting a flow of the fluid ina second direction opposite to said first direction; and (b) selectivedisabling means for selectively disabling said check valve to preventfunctioning of said check valve to reduce the pressure to be applied tosaid front wheel brake cylinder; (c) a load-sensing proportioning valvedisposed between said hydraulic pressure source and said rear wheelbrake cylinder, said load-sensing proportioning valve including aload-sensing proportioning valve applying to said load acting on thevehicle, said load-sensing proportioning valve applying to said rearwheel brake cylinder the pressure generated by said pressure source whensaid pressure generated by said pressure source is lower than athreshold level, and reducing said pressure generated by said pressuresource and applying the reduced pressure to said rear wheel brakecylinder after said pressure generated by said pressure source exceedssaid threshold level, said threshold level increasing as said loaddetected by said load-sensing member increases,said selective disablingmeans disabling said check valve when said load detected by saidload-sensing member is smaller than a predetermined load value, wherebysaid distribution control device controls said distribution of saidfront and rear wheel braking forces according to said first distributionpattern when said load detected by said load-sensing member is smallerthan said predetermined load value, and according to said seconddistribution pattern when said load is not smaller than saidpredetermined load value.
 13. A braking system according to claim 12,wherein said selective disabling means comprises:(a) a by-pass passagewhich is in parallel connection with said check valve and whichby-passes said check valve; (b) a solenoid-operated shut-off valvedisposed in said by-pass passage; (c) a load-sensing switch responsiveto said load-sensing member and generating a first signal when said loaddetected by said load sensing member is smaller than said predeterminedload value and a second signal when said load detected by said loadsensing member is not smaller than said predetermined load value; and(d) a controller opening said shut-off valve in response to said firstsignal to thereby disable said check valve, and closing said shut-offvalve in response to said second signal to thereby enable said checkvalve to reduce the pressure to be applied to said rear wheel brakecylinder with respect to the pressure generated by said pressure source.14. A braking system according to claim 1, wherein said distributioncontrol device selects said second distribution pattern in at least oneof a first condition in which a load acting on the vehicle is largerthan a predetermined load value, and a second condition in which thebraking system is operated in an anti-lock pressure control mode.
 15. Abraking system according to claim 1, wherein said front and rear brakesinclude, respectively, a front wheel brake cylinder and a rear wheelbrake cylinder which are supplied with a pressurized working fluid, andwherein said distribution control device selectively establishes saidfirst and second distribution patterns of said front and rear wheelbraking forces by controlling a front wheel braking pressure in saidfront brake cylinder and a rear wheel braking pressure in said rearwheel brake cylinder,and wherein said first distribution pattern of saidfront and rear wheel braking forces is established by a firstdistribution pattern of said front and rear wheel braking pressures inwhich said front and rear wheel braking pressures change such that saidfront and rear wheel braking pressures are equal to each other, whilesaid second distribution pattern of said front and rear wheel brakingforces is established by a second distribution pattern of said front andrear wheel braking pressures in which said front and rear wheel brakingpressures change such that said rear wheel braking pressure is higherthan said front wheel braking pressure.
 16. A braking system for brakinga motor vehicle by operation of a front and rear brake for a front and arear wheel of the vehicle, respectively, said braking systemcomprising:a distribution control device to control a distribution of afront wheel braking force and a rear wheel braking force which areproduced by said front and rear brakes, respectively, and which areapplied to said front and rear wheels, said distribution control devicecontrolling said distribution according to a selected one of a firstdistribution pattern and a second distribution pattern, each of saidfirst and second distribution patterns being a relationship between saidfront and rear wheel braking forces such that said rear wheel brakingforce defined by said second distribution pattern is larger than thatdefined by said first distribution pattern at least when said rear wheelbraking force is being increased.
 17. A braking system according toclaim 16, wherein said distribution control device selects said seconddistribution pattern in at least one of a first condition in which aload acting on a vehicle is larger than a predetermined load value, anda second condition in which the braking system is operated in ananti-lock pressure control mode.
 18. A braking system according to claim17 of a diagonal or X-crossing type for a four-wheel motor vehicle,having two pressure application sub-systems which are connected torespective two mutually independent pressurizing chambers of a mastercylinder, each of said two pressure application sub-systemsincluding:(a) a front brake cylinder passage connecting a correspondingone of said two pressurizing chambers of said master cylinder and afront wheel brake cylinder of said front brake, (b) a rear brakecylinder passage connecting said front brake cylinder passage and a rearwheel brake cylinder of said rear brake (c) a master cylinder cut valvein the from of a normally-open shut-off valve disposed in a portion ofsaid front brake cylinder passage between said master cylinder and apoint of connection of said front and rear brake cylinder passages, saidmaster cylinder cut valve being closed when the braking system is in ananti-lock pressure control mode, and opened when the braking system isnot in said anti-lock pressure control mode, (d) an intermediate valvein the form of a normally-open shut-off valve disposed in said rearbrake cylinder passage, (e) a reservoir passage connected at one ofopposite ends thereof to a portion of said rear brake cylinder passagebetween said intermediate valve and said rear wheel brake cylinder, (f)a reservoir connected to the other end of said reservoir passage, (g) apressure reducing valve in the form of a normally-closed shut-off valvedisposed in said reservoir passage, (h) a pump passage connected at oneof opposite ends thereof to said reservoir and at the other end to aportion of said rear brake cylinder passage between said intermediatevalve and a point of connection of said front and rear brake cylinderpassages, (i) a pump disposed in said pump passage for delivering aworking fluid from said reservoir to a portion of said each pressureapplication sub-systems, and (j) a controller operable in said anti-lockpressure control mode for controlling said master cylinder cut valve,said intermediate valve, said pressure reducing valve and said pump toeffect an anti-lock pressure control operation for controlling pressuresof said fluid in said front and rear wheel brake cylinders in ananti-lock manner,and wherein said distribution control device comprisessaid controller, and a check valve device disposed in a portion of saidrear brake cylinder passage between the point of connection of saidfront and rear brake cylinder passages and a point of connection of saidrear brake cylinder passage and said pump passage, said check valvedevice comprising a first check valve and a second check valve, saidfirst check valve permitting a flow of the fluid therethrough in a firstdirection from said pump toward said front wheel brake cylinder afterthe pressure generated by the pump becomes higher than the pressure insaid front wheel brake cylinder by more than a predetermined difference,and inhibiting a flow of the fluid therethrough in a second directionopposite to said first direction, said second check valve permitting aflow of the fluid therethrough in said second direction and inhibiting aflow of the fluid therethrough in said first direction.
 19. A brakingsystem of a diagonal or X-crossing type for braking a four-wheel motorvehicle by operation of a front and a rear brake for a front and a rearwheel of the vehicle, respectively, said braking system including twopressure application sub-systems which are connected to respective twomutually independent pressurizing chambers of a master cylinder, each ofsaid two pressure application sub-systems comprising:(a) a front brakecylinder passage connecting a corresponding one of said two pressurizingchambers of said master cylinder and a front wheel brake cylinder ofsaid front brake; (b) a rear brake cylinder passage connecting saidfront brake cylinder passage and a rear wheel brake cylinder of saidrear brake; (c) a master cylinder cut valve in the form of anormally-open shut-off valve disposed in a portion of said front brakecylinder passage between said master cylinder and a point of connectionof said front and rear brake cylinder passages, said master cylinder cutvalve being closed when the braking system is in an anti-lock pressurecontrol mode, and opened when the braking system is not in saidanti-lock pressure control mode; (d) an intermediate valve in the formof a normally-open shut-off valve disposed in said rear brake cylinderpassage; (e) a reservoir passage connected at one of opposite endsthereof to a portion of said rear brake cylinder passage between saidintermediate valve and said rear wheel brake cylinder; (f) a reservoirconnected to the other end of said reservoir passage; (g) a pressurereducing valve in the form of a normally-closed shut-off valve disposedin said reservoir passage; (h) a pump passage connected at one ofopposite ends thereof to said reservoir and at the other end to aportion of said rear brake cylinder passage between said intermediatevalve and a point of connection of said front and rear brake cylinderpassages; (i) a pump disposed in said pump passage for delivering aworking fluid from said reservoir to a portion of said each pressureapplication sub-system; (j) a controller operable in said anti-lockpressure control mode for controlling said master cylinder cut valve,said intermediate valve, said pressure reducing valve and said pump toeffect an anti-lock pressure control operation for controlling pressuresof said fluid in said front and rear wheel brake cylinders in ananti-lock manner; and (k) a check valve device disposed in a portion ofsaid rear brake cylinder passage between the point of connection of saidfront and rear brake cylinder passages and a point of connection of saidrear brake cylinder passage and said pump passage, said check valvedevice comprising a first check valve and a second check valve, saidfirst check valve permitting a flow of the fluid therethrough in a firstdirection from said pump toward said front wheel brake cylinder afterthe pressure generated by the pump becomes higher than the pressure insaid front wheel brake cylinder by more than a predetermined difference,and inhibiting a flow of the fluid therethrough in a second directionopposite to said first direction, said second check valve permitting aflow of the fluid therethrough in said second direction and inhibiting aflow of the fluid therethrough in said first direction.